Zucchini & Squash Casserole

Place oven rack in middle position; preheat to 375°F. Melt 1/2 tablespoon butter in a large nonstick skillet over medium-high heat. Add zucchini and 3/4 cup onion; cook, stirring often, until the center of the zucchini is very tender and translucent, 10 to 12 minutes. Transfer the mixture to a colander set over a bowl. Repeat with yellow squash and the remaining 1/2 tablespoon butter and 3/4 cup onion. Add the squash mixture to the colander with the zucchini; let drain for 5 minutes. Using a rubber spatula, gently stir and press down on the zucchini-squash mixture to extract as much liquid as possible without disfiguring the squash; discard liquid.

Whisk eggs, mayonnaise, pesto and salt together in a large bowl. Gently fold in the zucchini-squash mixture and mozzarella. Spoon into a broiler-safe 2-quart baking dish.

Combine panko and Parmesan in a small bowl; sprinkle evenly over the casserole. Lightly coat the panko mixture with cooking spray. Bake until golden brown in spots, about 30 minutes.

Set oven to broil. (Do not remove casserole from oven.) Broil until the topping is golden brown all over, 2 to 3 minutes. Let stand for 5 minutes before serving.