Step 1

Preheat oven to 350°F. Lightly coat an 7-by-11-inch baking dish with cooking spray. Set aside.

Step 2

Melt 1 tablespoon butter in a large nonstick skillet over medium-high heat. Add half of the zucchini and half of the onion; cook, stirring often, until the center of the zucchini is just tender and the liquid has evaporated, about 8 minutes. Transfer the mixture to a colander set over a bowl. Repeat the cooking process with the remaining zucchini and onion. Let the vegetables drain in the colander for 5 minutes.

Step 3

Combine eggs, yogurt, mayonnaise, thyme, pepper and salt in a large bowl. Gently fold in the zucchini-onion mixture. Spoon evenly into the prepared baking dish. Sprinkle evenly with cheese.

Step 4

Microwave the remaining 2 tablespoons butter in a medium microwaveable bowl on High until melted, about 25 seconds. Add crushed crackers to the melted butter and toss together until combined; sprinkle the topping evenly over the casserole. Bake until golden brown, about 20 minutes. Let stand for 5 minutes before serving.