Warm a cast iron skillet over medium heat. Crumble sausage into pan and cook until fully browned. Add the sliced bell peppers (and the optional onions) and sauté for about 3 minutes, just until they start to soften.
In a large bowl, whisk together 8 eggs, salt, and pepper. (You can totally increase this to 10-12 eggs for a larger frittata.)
If needed, add a little oil or butter to help coat the skillet. (If your cast iron is well seasoned, this isn't needed, but it is always a helpful insurance plan to keep your frittata from sticking.)
Make sure the sausage and peppers are evenly distributed in the pan and then cover with the whisked eggs. Cook for 1 more minute on stove, then transfer to the preheated oven.
Bake on 350ºF for 30-35 minutes or until fully cooked through.
Notes
Divide this recipe into muffin tins for Paleo Egg Muffins!Increase eggs to 10-12 to make a larger frittata - just increase the cooking time by 5 or so minutes.