Preheat oven to 325 degrees. Grease the bottom and sides of a glass pie pan with the butter.
In a large skillet, heat onion, garlic, mushrooms, and plant-based sausage in olive oil approximately 4 minutes or until sausage is browned and mushrooms are soft. Stir in the salt and pepper. Set aside.
In a blender, add firm and silken tofu, soy sauce, and nutritional yeast and blend. Add to the sausage mixture and combine well. Pour all into the pie pan.
Bake for 45 minutes. Let cool for 10 minutes allowing frittata to become firm.