In an 11” skillet over medium-high heat, preheat oil. Add whipped eggs, sausage and mozzarella to the pan, pulling edges in with a spatula as it starts to setup. Distribute ricotta in clumps evenly across the frittata, pressing down into the egg mixture. Reduce heat to medium-low and cover for 10-15 minutes. Uncover. Swirl the pan gently to check consistency of the frittata. If mostly firm, cover pan with lid, plate or round pizza pan (the key is that it needs to be at least as large as the skillet and have no sides). Hold firmly against the top of the skillet and carefully flip the frittata over onto the flat surface. Slide the frittata back into the skillet and brown on bottom side for an additional 3-4 minutes. Slide onto serving dish and let stand 10 minutes to cool before service.