French lentils, also known as lentils de Puy, have a firm texture that holds up well in salads. Prep the rest of the ingredients while the lentils cook, then toss greens and lentils together. Serve warm with a baguette and cheese.
Add the lentils to a medium saucepan with 6 cups water, the garlic, thyme, and 1 tsp. salt. Bring to a boil, then reduce the heat to medium-low and simmer until the lentils are tender, about 20 to 25 minutes. Drain well and discard the garlic cloves.
Add the spinach, apple, Parmigiano, and shallot to a large bowl with the olive oil and vinegar. Add the warm lentils and toss well.
Season with salt and pepper to taste, sprinkle with walnuts, and serve.