This is a recipe that can be easily made vegetarian (using vegetable rather than chicken stock). The smoked mozzarella gives the lentils an almost bacony taste, but fresh plain mozzarella also works well if smoked mozzarella isn't available. The dish can be either an entree or a side salad, and can be served hot, at room temperature, or refrigerated. If it is refrigerated, add the walnuts, mozzarella, and thyme just before serving.
Bring the broth and lentils to a boil, reduce heat to simmer, cover, and cook until the lentils are tender but still firm, about 30 minutes. Remove from heat, let sit for 10 minutes in the water, and drain. Put into a large bowl.
While the lentils are cooking, mix the lemon juice, mustard, olive oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper together in a bowl or jar.
Add the vinaigrette to the warm lentils and toss, set aside.
Roast the red pepper by holding it over the open flame of a gas burner with a pair of tongs, turning constantly. Alternatively, rub the pepper with a little oil and put it under a broiler, turning constantly. When the pepper has completely charred, put into a small paper bag and allow to steam (10 minutes). When finished steaming, rub off the charred skin, slice the pepper open, and remove the seeds and white rib. Then dice the pepper into 1/2” inch pieces. This step can be done ahead of time.
When the lentils are warm or at room temperature, add the red pepper, walnuts, onion, mozzarella, and thyme. Add 1/2 teaspoon salt (or more to taste) and 1/4 teaspoon pepper. Toss and serve.