Most mornings, 49-year old Michèle Jean-Bart takes her two dogs and bikes to work, cycling about a mile over the pancake-flat countryside where she lives on the Île de Ré, near La Rochelle on France’s western coast. She arrives at her salt marsh and begins work early, often around 6 a.m.

In spring, she’ll be prepping the pools of salty water, removing as much mud as she can and readying them for the summer harvest. As the weather grows sunnier, likely in June, she’ll begin retrieving crystals as the water evaporates each day — or, at least, if the weather cooperates. Too much rain, as there was this year, and the watery puddles can turn brackish and won’t evaporate well.

“During summer, one aire saunante — that’s the square crystallizing pan where we collect the salt from — produces around 40kg [about 90 pounds] of coarse salt every two days,” Ms. Jean-Bart said.

Rhétais salt is graded after it’s harvested; most of that processing takes place in the fall. It will be sorted between two main grades. The finest is fleur de sel, which crystallizes in a flimsy film on the surface of the water and is best used as a seasoning at the table.

Gros sel is coarser, better for cooking and forms on the bottom of the pans, or pools. It is also dried and ground to produce a finer salt that can be used in much the same way. As the weather gets colder, Ms. Jean-Bart can rest; in winter, she’ll focus on tending the marshlands so they don’t become overgrown.

That yearly rhythm is the same one that sauniers have followed since salt making emerged on the islands in the Middle Ages. It was once a thriving industry, peaking in the 19th century when more than 1,000 workers made salt for France’s booming kitchens. Now, Ms. Jean-Bart said, only 100 or so remain here. The salt-making industry stands in stark contrast with the modern perception of the Île de Ré. It has emerged as a summer getaway for bourgeois bohemians from Paris. They take the roughly three-hour train ride from the capital on summer weekends to their cottages here.

It’s easy to see why: It’s a charming country hideaway, dotted with villages, and the landscape is quilted with bike paths, which people use to get around the about 19-mile-long island (the Tour de France included the island on its circuit this year for the first time).

The eastern end of the island, which is closest to the mainland and the bridge that links the two, is most developed. That’s where the few thousand year-round residents tend to cluster. The western tip, though, is much fancier, especially once past the village of La Couarde-sur-Mer; the cottages there are more prized for their seclusion and exclusivity. Passing La Couarde-sur-Mer is also when the landscape changes, and the acres of salt pans begin, stretching across the horizon and shimmering in the sun.

The best Rhétais salt makers, like Ms. Jean-Bart, are members of the Coopérative des Sauniers, which oversees production here and markets the fleur de sel in particular. Many of them, like her, are from local families; some current practitioners were drawn here by the lure of the salt. Nicolas Decis, 50, is one of them.

He was born in Normandy and worked at the salt museum here, Écomusée du Marais Salant, 20 years ago. He said it was hard to break into the craft because the producers were leery of the museum and rarely visited so he could talk to them. Mr. Decis spent days watching the museum’s videos of them working to try to glean tips for the career he hoped to follow.

Then the rain came — 2000 was a bad year for the harvest, much like 2020. The dreary weather meant that veteran salt makers had time to teach him, and within two years he was setting up his own salt marsh.

“I became totally addicted to the salt marshes and salt production then,” Mr. Decis said. “It’s a totally hand-harvested product, renewable and organic, and it’s produced by small enterprises.”

The only major change in technique over the centuries, he said, are the tractors that transport salt from the pans: In the past, they would have been wheelbarrows.

“The salt marsh is not my property; I’m just at its service,” he said. “Walking on the same clay path doing the same gesture as the guys in the 13th or 14th centuries is one of the reasons I love my job so much. One of my favorite souvenirs, or memories, was an evening when me and my older son were harvesting the coarse salt, while my second son was working on our fleur de sel. It all makes our salt very special.”

The best place to find an assortment of salt from producers like Mr. Decis is at the Cabane des Sauniers, the store operated by the cooperative in Ars-en-Ré, though every village on the island will sell jars from various producers.

Salt is often sold by the kilogram on the island, with the coarser grade costing an average of 1.50 euros per kilo (about $2 for 2.2 pounds) and fleur de sel at €20 or so for the same amount, Ms. Jean-Bart said. The cooperative runs an online store where the salt is sold in more pantry-friendly sizes. In New York, the Meadow in the West Village sells two-ounce jars of gros sel for $9 and fleur de sel in 2.5-ounce jars online for $18.

Aurélien Massé, the sommelier at Frenchie, a restaurant group in Paris, grew up on the island. Look, above all, for a rosy tinge to the fleur de sel, he advised, because that’s a distinctive marker of Rhétais salt.

Though the cooperative’s museum offers a comprehensive explainer of the salt-making process, consider stopping by a saunier and asking about it directly; buy salt straight from one of them, too. Mr. Massé recommends Les Salicorniers in Ars-en-Ré, where the salt is also sold blended with herbs.

“Purists will tell you that’s not real salt, but on a barbecue or with fish, it’s cool to have herbs, too,” he said.

Ms. Jean-Bart is more of a traditionalist and doesn’t add herbs.

She recommends simply putting a chicken or some beets in an oven dish, then covering it with gross sel Rhétais and baking it for one hour per kilogram.

“I grew up with salt from my island,” she said, “so I use it on all my recipes.”