Lentilles du Puy are not your ordinary run-of-the-mill green lentils even by French standards. This particular variety grows in the rich volcanic soil of the Haute-Loire which give them a nutty flavor with a hit of minerality.
Recipes for preparing these little gems are numerous both in print as well as in cyberspace. Having read many of them, one note from the magazine Cooks Illustrated reminded me that when cooking legumes, salt should not be added until the end as it inhibits their cooking and they remain too firm.
However with lentils, they are very tender and can easily become mushy. Therefore, salt should be added at the beginning of the cooking process to prevent the “mushiness” factor. Taking their suggestion and adding salt at the beginning, the lentils cooked beautifully keeping their shape and becoming ever so tender.
The majority of recipes also call for sautéing the vegetables in a large saucepan, adding the lentils and cooking everything in copious amounts of water. When the lentils are tender to the tooth, drain off the water and rinse. “What, I asked myself, ”1) why would one cook vegetables for 35-40 minutes and 2) pour off all that flavor?”
I can understand using water if that’s the only liquid available. However, I was using homemade duck stock and what wasn’t absorbed by the legumes, I didn’t want to pour down the drain.
I knew what I wasn’t going to do, but I didn’t have a Plan B when I started cooking these delightful gems. I did, however, know that I had to come up with something that didn’t involve the drain.
After about 20 minutes at a gentle simmer, the liquid was evaporating and they were cooking nicely. I tasted a few to determine how they were coming along. They weren’t quite there yet so I added another cup of stock. The trick, I realized was very much like cooking risotto, keep enough liquid to allow the lentils to cook, but not to have too much liquid left at the end.
When the lentils were almost tender and there was barely enough liquid, I covered the cooking vessel with a lid. This allowed them to continue cooking without adding any more liquid. When they are just tender, I removed the lid and allowed the remaining liquid to cook off. Using a lid was the key and to taste, taste, taste!
The French are all about the “bouquet garni”, so before I started cooking I gathered a fewherbs and tied them inside a couple of leek leaves.
Also, I implore you to use the best sherry vinegar, mustard, and olive oil your pocketbook will allow. Quality ingredients do make a difference in the end, especially when adding to a simple dish like lentils or for vinaigrette.
I served the lentils alongside a link of lamb sausage and a frisée salad with hazelnut vinaigrette. You could also serve them with duck confit or pâté and by all means enjoy a crusty baguette.
Bouquet garni (a couple of the large green leek leaves, rosemary sprig, 4 thyme sprigs, 4 parsley sprigs, 3 bay leaves), tied with cotton twine
10 ounces thick cut (1/4-inch if possible) bacon or lardon
1 medium leek
2 ½ cups (500 grams) Lentilles du Puy 
1 medium onion, peeled and cut into ½-inch dice
4 medium carrots, peeled and cut into 1/2-inch dice
2 celery ribs, cut into 1/2-inch dice
3 medium cloves garlic, minced
1/2 teaspoon kosher salt or to taste
1 teaspoon freshly ground black pepper
1/3 – 1/2 cup extra virgin olive oil
1/4 cup minced fresh parsley leaves
4 or more cups duck or chicken stock, preferably homemade
1 – 2 tablespoons sherry vinegar
2 tablespoons whole grain mustard
Remove the outer leaves of the leek where it turns from white to green, reserving two large ones.
Lay the rosemary, thyme and parsley sprigs, and bay leaves in a leek leaf. Top with another leek leaf and tie in a bundle with cotton twine.
Cut the bacon in ¼-inch strips across the grain. Set aside.
With the remaining leek, remove the root and cut in half length-wise and then into ¼-inch slices. Add leeks to a bowl and cover with cold water. Swish the leeks around to rinse, allowing any sand to fall to the bottom of the bowl. Then, carefully lift out the leeks to drain in a sieve, leaving the sand and water behind.
In another bowl rinse the lentils, looking for and discarding any pebbles or other objects that shouldn’t be there. Transfer the lentils and bouquet garni to a large saucepan, Dutch oven or French cocotte. 
Add enough stock, to cover the lentils by a good 1/2-inch. Bring to a simmer and cook stirring often. The total cooking time should be between 30-40 minutes.
While the lentils are cooking, heat a medium sauté pan and add the bacon. Cook until the bacon is crispy. Remove the bacon to a plate lined with paper toweling. Pour off all but 2 tablespoons of fat.
Heat the pan again and sauté the onion, carrots, celery, and leeks until carrots are barely tender. Add the garlic and continue to cook another 30 seconds. Remove from heat and set aside.
As soon as the lentils are cooked, turn off the heat. Add the sautéed vegetables, sherry vinegar and mustard. Stir to combine. Serve warm or at room temperature.