2 Tablespoons olive oil or butter
1/2 onion, sliced (optional)
Salt and pepper to taste
4 to 6 cups of any chopped or slice raw or barely cooked vegetables
1/4 cup fresh basil or parsley leaves, or 1 teaspoon chopped fresh tarragon or mint leaves, or any other herb
3 eggs
1/2 cup freshly grated Parmesan cheese (optional)
Put olive oil or butter in a 10 inch skillet ( preferably nonstick or well-seasoned cast iron) and turn heat to medium. When fat is hot, add onion, if using, and cook, sprinkling with salt and pepper, until it is soft, 3 to 5 minutes. Add vegetables, raise heat and cook stirring occasionally, until they soften, from a couple of minutes for greens to 15 minutes for sliced potatoes. Adjust heat so vegetables brown a little without scorching. (With precooded vegetables like asparagus and squeezed dry spinach, just add them to onions and stir before proceeding.)
When vegetables are nealy done, turn heat to low and add herb. Cook, stirring occasionally, until vegetables are tender.
Meanwhile, beat eggs with some salt and pepper, along with cheese if you are using it. Pour over vegetables, distributing them evenly. Cook, undisturbed for a minute or so. Put into a 350 degree oven for about 10 minutes until set. Slide frittata onto a plate and cut into wedges.