Place a nonstick skillet over moderate heat. When skillet is hot, add butter, mushrooms, garlic and shallot. Season with salt and pepper and saute 5 minutes. Transfer mushrooms, garlic and shallot to the food processor. Pulse to grind the mushrooms and transfer to the mixing bowl, adding the processed mushrooms to the spinach. Add ricotta and grated cheese to the bowl and the nutmeg. Stir to combine the stuffing. Return your skillet to the stove over medium high heat.