Heat oven to 350 degrees. In a cast iron skillet, heat the olive oil over medium heat. Add the sausage to the skillet and cook about 5-7 minutes. Add the onions to the skillet and cook until they are translucent. Add the spinach and cook about 1-2 minutes. Add salt and pepper to taste. Beat the eggs and the milk together. Add the egg mix to the skillet; top it with cheese and tomato slices. Using potholders or oven mitts, remove the cast iron skillet from the stovetop and put it in the oven. Cook about 8-10 minutes until the eggs are firm. Sprinkle some cayenne pepper on top and enjoy!