Preheat your oven to 300°F (150°C).
In a large Dutch oven or heavy-bottomed pot, combine the salt, pepper, crushed garlic, thyme, and bay leaf. Mix well.
Pat the duck legs dry with a paper towel, then add them to the pot and coat them in the spice mixture.
Cover and place in the fridge for at least 2 hours or overnight.
In a skillet, heat the duck fat or vegetable oil over medium-high heat.
Remove the duck legs from the fridge and wipe off any excess spice mixture.
Carefully place the duck legs in the skillet and cook for 5 minutes on each side until golden brown.
Transfer the duck legs to a baking dish and cover with a layer of cheesecloth.
Bake in the oven for 1 hour and 15 minutes until the meat is fork-tender.
While the duck is cooking, prepare the lentils. In a pot, heat a tablespoon of the duck fat or vegetable oil over medium heat.
Add the chopped onion, minced garlic, diced carrots, and celery. Cook for 5 minutes until the vegetables are softened.
Add the lentils, chicken or vegetable stock, tomato paste, and Dijon mustard. Stir to combine.
Bring to a boil, then reduce the heat and let simmer for 20 minutes until the lentils are cooked.
Season with salt and pepper to taste.
To serve, place a spoonful of the lentils on a plate and top with a duck leg. Garnish with chopped parsley.
Serve hot and enjoy!