Veggie-filled frittatas are delightfully quick, versatile, and inexpensive. Here, cheerful summer squash is paired with the summery flavors of pesto -- basil, parmesan, and pine nuts -- though you could substitute a spoonful of prepared pesto if you have some handy. If your garden or market is spilling over with other vegetables, use them instead, cooked and added to the eggs with an herb and cheese of your choice.
Slice the squash diagonally into very thin slices, then stack the slices and slice them into skinny matchsticks (if you're pressed for time, you can quarter the squash lengthwise and slice them).
Heat 1 tablespoon of the olive oil in a 10-inch skillet over medium-high heat, and add the squash with 1/2 teaspoon salt (less if you're using table salt). Sauté the squash, stirring and flipping occasionally, until they are wilted and browning, around 10 minutes.
While the squash cooks, beat the eggs in a bowl, then beat in the parmesan, basil, pine nuts, and 1/2 teaspoon salt. Scrape the cooked squash into the eggs, then return the pan to low and add the butter and the remaining tablespoon of olive oil.
Quickly pour in the eggs, then cover the pan and cook as slowly as possible, covered, until the eggs are mostly set -- around 10 minutes. If your skillet is oven-proof, slide it under the broiler for a minute or two to set the top; if it's not, slide the frittata onto a plate, flip it onto another plate, and slide it back into the pan to finish cooking for a minute or two.
Meanwhile, in a very small pot or pan, boil the vinegar over medium-high heat with 1/2 teaspoon salt until it is reduced to a syrupy tablespoon (around 5 minutes). Brush this glaze over the finished frittata and serve hot, warm, or at room temperature.