Place the sweet potato and sausage balls onto the baking sheet and cook in the oven for 10 minutes. Add the onion, pepper and mushrooms and return to the oven for a further 10 minutes. Scatter over the wilted spinach. In a jug mix together the eggs and milk and season generously. Tip into the dish to coat all the cooked vegetables and sausage balls. Scatter over the cheese and bake for 15 minutes.