To make filling, place the lentils and broth in a pot and heat to a boil. Simmer, covered, for about 20 minutes until the lentils are tender and the broth has been absorbed.
Lightly oil a large skillet, and preheat it over medium heat. Add the mushrooms, onion, celery and garlic and stir to mix. Cover and cook, stirring occasionally, for about 3 minutes until the mushrooms begin to brown. Add a few teaspoons of water or broth if the skillet seems too dry.
Remove skillet from the heat. Stir in the rice and lentils, then the cheese. Stir in the nuts and set aside.
Cut the tops off the peppers and remove seeds and membranes. Divide the filling among the peppers and replace the pepper tops.
To make the sauce, combine the thyme, salt, and tomato sauce. Place the peppers upright in a deep casserole dish, and pour the sauce around the peppers. Cover the pan with aluminum foil and bake at 350 degrees F for about 1 hour, until the peppers are tender.
Makes 4 servings. Per serving: 360 calories, 3.2 g. total fat, 65 g. carbohydrate, 17 g. protein, 429 mg. sodium.