Chicken and Bacon Frittata

I absolutely love frittatas. For one, you can eat them for breakfast or dinner, and you can change up the ingredients to make it whatever you want. Secondly, I'm terrible at making omelets, I can never get them to flip just the way I want them, and they turn out really messy looking. Basically, a frittata is an open-faced omelet. I make many variations, but the following recipe is one I made recently, so for you meat-eaters out there, you'll especially love this one. Remember, you can make it whatever you want, so feel free to substitute the ingredients to what you'll like! Frittatas are also a great way to use left-over ingredients that you have in the fridge! The following recipe feeds four, it's a rather large frittata!
What you'll need:
 1 Chicken breast, cooked & shredded (I used a chicken breast from a rotisserie chicken)
  4 slices of bacon, cooked, drained, cooled & crumbled
  10 eggs
  4 tbsp milk
  1/4 cup grated parmesan cheese (Use Parmigiano-Reggiano if you have it!)
  1 Cup shredded cheese (I would highly recommend Gruyere cheese, as it melts wonderfully! I didn't have  
  any on hand when I made this, so  I use a mixture of provolone, mozzarella, and fontina).
  1/4 tsp dried oregano
  1/4 red onion
  salt & pepper
What to do:
1. In a large, oven-proof skillet, cook the onions with a bit of butter, 3-5 minutes over medium heat.
2. Add the chicken and bacon to the skillet just to heat through, a few minutes, while stirring occasionally.
3. Meanwhile, in a large mixing bowl, whisk the eggs, milk, parmesan cheese, oregano and salt & pepper.
4. Add the 1 cup of shredded cheese to the bacon, chicken and onions in the skillet, sprinkling all around the skillet. Next, add the egg mixture to the skillet, and let the eggs cook, lifting the sides occasionally with a spatula to let the eggs on top run to the sides and bottom of the skillet to cook.
5. When the bottom and sides are set, place the skillet in a pre-heated 400 degree oven on the top rack, and let it cook for an additional 10 minutes, until the top and edges begin to brown.
6. Let it rest for just a few minutes; the frittata will cut easily with a pizza cutter- serve with salsa if desired.