Stovetop and Broiler Method

1. Prepare Oven – Preheat the broiler and move a rack to the top third of the oven – 2nd from the top works well but make sure your skillet handle has clearance from the broiler heat element. If you need to use the 3rd rack from the top, add a minute or two to the cooking.
2. Cook Vegetables — heat olive oil in a 10″ –  11″ oven and boil-proof skillet on medium heat on the stovetop. Add diced onion and salt and saute until soft, about 3 minutes. Pour in chopped yellow squash pieces and cook for 1 – 2 minutes. Combine the mushrooms in the pan and cook until soft, about 3 minutes. Sprinkle the spinach over all the cooked vegetables and stir until wilted. Covering with a lid will speed up the process.
3. Cook Meat – If using sausage, place in a separate small skillet until cooked through and crumbly – about 5 minutes. Note: the chorizo is red from the spices and does not change color when cooking like other meats. Dry on paper towels or use a slotted spoon to separate the grease. Sprinkle the sausage over the vegetables in the skillet.
4. Prepare and cook eggs – Break the eggs into a medium mixing bowl and whisk until light, foamy, airy, and uniform in color. Add both kinds of cheese and salt and pepper, and mix gently to combine. Pour the eggs over the vegetables in the large skillet. Gently distribute any large feta chunks if they are located in one place—Cook for approximately 10 minutes on the stovetop. The center will still be runny.
5. Broil Frittata –  place the pan in the oven and under the broiler heat for 2 minutes. Pull the pan out and inspect for golden brown and firm in the middle. If not cooked or uniform in golden brown, place the pan in the oven and rotate the handle to the opposite side from the first cooking. Cook 1 minute more, being careful not to over-brown. The frittata is cooked when golden brown and firm to the touch, like a cake.
6. Serve – Allow the pan to cool before slicing into five slices***. This dish can be served immediately or within 1-2 hours at room temperature. To make ahead, Follow steps 1 -5 and reheat the frittata in the pan on the stovetop before slicing and serving. If made ahead longer than 1 hour, bring to room temperature and store covered in the refrigerator—either heat in the pan on the stove or heat individual pieces in the microwave.

Note: Stovetop and Oven Method – If you do not have a broiler, omit the boiling instructions and place the skillet with eggs and vegetables in the center of a 350-degree oven for 15 minutes or until eggs are firm.