Melt the butter in the bottom of the rice cooker. Add onions and cook, stirring occasionally, until onions are slightly translucent. Add mushrooms and continue cooking. Add spinach and cook until slightly wilted. Mix in eggs, cheese, salt and pepper and stir until combined. Place the lid on for 5 to 10 minutes, until the frittata appears fully cooked. The edges should be firm and the top and bottom almost set.
Use a spatula to transfer the frittata to a plate. Garnish with mozzarella cheese and enjoy!