Here's the 1, 2, 3 lowdown: crack four eggs in a bowl and whisk together with a splash of whole milk or cream. Set aside while you prep the frittata filling.
In this case, I sliced one yellow onion into thin strips, minced three cloves of fresh garlic, and diced two plum-sized tomatoes. I also roughly chopped some fresh, baby spinach and shredded about 3/4 cup of aged cheddar cheese.
Now heat up a heavy, 9" to12" oven-proof skillet over medium-high flame, and grease that puppy well with a tablespoon of butter and a few splashes of olive oil. Toss in the onion and saute, stirring, until it becomes slightly translucent. Now add the garlic and the spinach and saute for another 20 to 30 seconds, until the spinach starts to wilt slightly and the garlic perfumes the oil.
OK, you're ready for the eggs. Toss in the scrambled egg mixture and mix so the spinach and onion are distributed evenly throughout. Turn the broiler in your oven on and set a rack close to the top (maybe not in that order). Now add your cheese and your tomato to the egg mixture, pushing some of it below the custardy surface and leaving some cheese sprinkled over the top. Once the egg sets on the bottom, pop the whole skillet in the oven under the broiler. Leave the oven door open so you can watch the metamorphosis: You're looking for a browned, crispy top, while the inside of the custard should be just set. It will probably take two to four minutes tops. If you want to cheat a bit, you can use a knife to tell when the frittata is set. Just poke the knife into the center, and if it comes out clean (not wet) then you're golden.
The rest is even easier. Remove from the oven and slice your frittata into wedges. Serve with a big ol' dollop of creme fraiche (or sour cream) and a side salad of arugula laced with a touch of lemon and olive oil.
Again, you can make any type of frittata you like based on what you have in your fridge. Serve it to some friends, and they'll think you actually have some class. That's what I do anyway.