Preheat the oven to 400°F. Grease an 8″x11″ glass baking dish. Add butter to a medium-size sauté pan and sauté diced onion over medium heat until translucent, approximately 2-3 minutes.
Add the crumbled meatballs and cook until meatballs are halfway done, approximately 10 minutes. Remove from heat.
In a large bowl, beat the eggs with the cream. Season with salt and pepper, to taste. Add the tomato, spinach, parmesan cheese, and meatball/onion mixture to the bowl, and stir.
Transfer the mixture to the glass dish and place in the oven until the eggs are fully cooked and the casserole is firm all the way through, approximately 15-20 minutes.
Take out and serve immediately. Buon appetito!