Have a 10-inch, non-stick skillet ready on the stove (no heat yet), and a large, round platter next to the stove.
Place your cooked spaghetti (cold out of the refrigerator is fine) into a large mixing bowl.
Grate your cheeses, if needed, and add them to the bowl.
In a liquid measuring cup, beat your eggs with a fork. Add the eggs to the mixing bowl.
Add your salt and pepper to the bowl and stir everything with a fork to combine.
Heat your skillet on the stove over medium-low heat. Add your olive oil to the pan and spread it around with a silicone spatula.
Pour your spaghetti batter into the pan. Let the first side cook, uncovered, until the bottom of the frittata is deep, golden brown, and the edges are set. It could take around 20 minutes, depending on your stove. You can check it by using your spatula to gently lift up an edge and peak underneath. If your frittata seems like it will burn, lower the heat.
When the first side is done, turn off the heat. Place your platter face-down over your pan. Carefully carry the pan and platter to your sink so you can invert the frittata over the sink. Invert the frittata by holding the platter in place with one hand while using your other hand to flip over the pan. The frittata will now be on your platter, cooked side up.
Place your pan back on the stove. Gently slide the frittata into the skillet. Set the heat to medium low and let the other side of the frittata cook, uncovered, until golden brown. It could take around 10 minutes.
When your frittata is done, you may want to invert it again onto a clean platter, so the "prettier" side is facing up. (That's what I did for the photos in the post.)
Let the frittata rest for a few minutes before slicing it. Serve, warm, at room temperature, or even chilled.
Store leftover frittata covered in the refrigerator, for up to four days.
Pasta usually doubles in weight after it's cooked. If you're starting with leftover spaghetti, weigh it until you have about a pound.If you're starting with uncooked pasta, use ½ pound. Boil it in salted water, drain it, and mix 4 tablespoons of salted butter in it to prevent sticking. Then proceed with the frittata recipe.You can use leftover spaghetti that's been mixed with butter, marinara or pesto.If you have a smaller amount of pasta to work with, you can adjust the ingredients accordingly and use a smaller pan.The recipe as written also will work in a 12-inch skillet, but your frittata will be thinner.