Step 1

Position rack in upper third of oven; preheat broiler.

Step 2

Heat oil in a large broiler-safe nonstick or cast-iron skillet over medium-high heat. Add onion and zucchini and cook, stirring frequently, until soft, 3 to 5 minutes.

Step 3

Meanwhile, whisk eggs, salt and pepper in a bowl. Pour the eggs over the vegetables in the pan. Cook, lifting the edges to allow uncooked egg from the middle to flow underneath, until nearly set, about 2 minutes. Arrange mozzarella and sun-dried tomatoes on top and place the skillet under the broiler until the eggs are slightly browned, 1 1/2 to 2 minutes. Let stand for 3 minutes. Top with basil.

Step 4

To release the frittata from the pan, run a spatula around the edge, then underneath, until you can slide or lift it out onto a cutting board or serving plate. Cut into 4 slices and serve.