This recipe is an example of the way I make frittatas on the stovetop. Typically, I use whatever interesting bits of food I find in the fridge. Spinach and mushrooms are a wonderful combination that work with any type of cheese--from Swiss to blue--or none at all. Serve this hot, warm or cold for any meal of the day.
Heat the olive oil in a 12-inch skillet over medium-high heat. When it slides across the pan add the mushrooms and 1/8 teaspoon of the salt. Cook until the mushrooms expel all their moisture and begin to brown, about 6 minutes.
Meanwhile, beat the eggs with the remaining salt and the pepper plus a tablespoon of water. Whisk again.
When the mushrooms brown, add the spinach and cook until it wilts and the moisture evaporates. Taste for seasoning and transfer to a plate.
Reduce the heat to medium and add the remaining tablespoon of oil. When it slides across the pan, pour in the eggs all at once. Cook until the edges set, about 2 minutes.
Distribute the reserved filling plus the feta around the egg, then put on the lid, Turn the heat to low and set a time for 10 minutes. Allow the frittata to gently cook and steam without lifting the lid.
Check on the frittata by lifting the lid. If it is nearly set in the middle, turn off the heat, but replace the lid and let it steam for a few more minutes. It is done when the center is completely set and the egg has released from the sides of the pan.
Set the lid aside. Run a table knife around the edges. Put a dinner plate over the pan, and use hot pads to invert it onto the plate. Take a moment to let the weight of the frittata release itself from the pan. Set the pan aside and cut into large wedges for serving.