Caramelized Onion, Mushroom and Gruyere Frittata

1. In an oven-proof skillet, melt butter over medium heat. Add onions. Reduce heat to medium low and continue to cook until onions soften and become an even light brown color (20-30 minutes). Remove onions to a plate.
2. Meanwhile, in another skillet fry bacon over medium heat until crisp. Remove bacon from the pan (leave the drippings) and allow bacon to cool, then crumble it. In the same skillet, add mushrooms and cook over medium heat until soft. Sprinkle salt over mushrooms. Remove from heat.
3. In a large bowl, whisk together eggs, milk, and a dash of salt and pepper. Add mushrooms, onions, bacon, and 1/2 cup Gruyere. Stir to combine.
4. Pour egg mixture into the same skillet used to saute the onions. (You should have enough butter left in there to keep everything from sticking. If not, add more.) Cook over medium heat until eggs are partially set. The outside should look cooked with bits of uncooked egg throughout the middle.
5. Turn on the broiler of your oven. Sprinkle egg with remaining Gruyere then place skillet under the broiler. Broil for 2-3 minutes or until egg is fully cooked and cheese is slightly browned and bubbly. Sprinkle with Parmesan cheese if desired.