The point of the frittata is to use what you have, and the possibilities are limitless. Diana Shaw, in her book, Sweet Basil, Garlic, Tomatoes, and Chives, has recipes for frittatas made of leftover pasta, old bread, and a variety of vegetables. Use what's in season, what's in the bottom of your crisper, what you can't get enough of. I like a frittata seasoned simply; salt and pepper are almost always enough.
Heat, over medium heat, a wide pan that can go from stovetop to oven (cast iron or stainless steel preferred). Pour a little butter or olive oil into the pan; add the sliced onions. Sauté until translucent.
Add the potato slices and greens; sauté, stirring often, until just cooked through.
Mix the eggs and crème fraîche and salt and pepper to taste; pour over the vegetables.
Sprinkle in the cheese and cook for a few minutes, until the eggs on top of the frittata are just losing their liquid nature. Slide the frittata into the oven and cook until barely browned.
Remove from the oven; cut into pie-shaped wedges; serve.