Heat the broth in a saucepan over medium heat. Add the mushrooms, miso, umami sauce, thyme, 2 tablespoons of nutritional yeast, and black pepper to taste. Cook, stirring, for 5 minutes, or until the mushrooms are soft. Transfer to a blender or food processor. Add 1/2 cup of the cooked lentils and blend until smooth. Add up to 1/2 cup of additional broth to make the gravy smoother, as desired. Combine the gravy with the remaining 1 1/2 cups of lentils, add to the steamed veggies, and stir. Set aside.