Combine the buckwheat groats and cracked wheat in a bowl and set aside. Saute the onion, celery and green pepper in the oil until light brown, seasoning with the onion and garlic powder. Add the mushrooms and brown. Stir in the tomatoes and stir for 2 minutes. Add the buckwheat mixture and mix well. Add the remaining ingredients and mix well. Put into a lightly oiled baking dish and bake for 30 minutes in a preheated 180ºC . oven.