1. To make sauce, stir garlic and parsley into yogurt and set aside. 2. Cook peppers in boiling water for about 10 minutes. 3. Meanwhile, heat oil and cook mushrooms, onions, celery and thyme together for about 3 minutes. 4. Stir in the cooked millet and seasoning, then heat through thoroughly. 5. Drain pepper halves, then fill with the mushroom mixture. 6. Serve immediately with the garlic and yogurt sauce. Note: Alternatively, serve peppers with a tomato sauce to suit vegans.