Left-overs Frittata
Preheat oven to 350° F.
In a large bowl, mix the eggs and milk vigorously with a whisk. Stir in the meat, vegetables and cheese. Season to taste.
Melt butter in a 10 inch cast iron skillet (or other oven-safe skillet) and tilt the skillet to coat the bottom with melted butter. Pour the egg mixture into the skillet, distribute evenly. Cook over medium-high heat until the edges begin to set. As the eggs are cooking, use a spatula to pull the edges of the frittata away from the pan and let uncooked egg fill in the space. After 3 to 5 minutes, the eggs will still be wet in the middle of the skillet - remove the skillet from the stove and place in the preheated oven.
Cook the eggs in the oven for 8 to 10 minutes. Check for doneness by cutting a slit in the middle of the frittata with a knife or the spatula blade. The frittata is done when liquid eggs do not run into the slit. Do not overcook - the eggs will continue to cook after the skillet comes out of the oven. Remove the skillet from the oven and allow it to cool for 5 minutes.
Serve from the skillet or loosen the edges with a spatula and slide the frittata onto a serving plate.