In a 4 quart oven-safe sauté pan over medium heat, heat the olive oil. When the oil is hot add in the chicken-apple sausage and cook for 5 Minutes or until both sides are caramelized.
Use the water (or stock) to deglaze the pan and scrape up any caramelized bits from the bottom of the pan. Add in the diced pumpkin and cook for 5 Minutes.
Add in radishes and red onion and cook for another 5 Minutes. Then, working in batches if needed add in the spinach and cook until it has wilted and there is very little water left in the bottom of the pan.
In a bowl whisk the eggs and milk together and pour over the sausage and vegetables. Don't stir. Top with the parmesan cheese.
Bake for 25 Minutes or until the cheese has browned and the eggs are set. Remove and rest for 2 Minutes before serving.
If you stir the eggs into the casserole it can cause scorching on the bottom and the texture to change. As hard as it is, resist the urge to stir them in! You're not making scrambled eggs, you're making a breakfast casserole.