Though its peak season is summer, zucchini is available year-round—just be sure to pick out the best-looking squashes. They should be firm (not spongy), vibrant in color, and free of blemishes. Cooking the squash before stirring it into the egg mixture helps eliminate some of the moisture, and adds a nice crisp-tender texture to the creamy eggs and cheese. Though it bakes primarily in the oven, finishing the frittata under the broiler gives the top and outer edges a bit of crispness. Serve the dish with toast and fruit for breakfast, or with sautéed mushrooms for dinner.