Begin by thinly slicing the zucchini and yellow squash into ⅛-inch rounds. Place them in a colander, sprinkle with salt, and toss. Let sit for 20–30 minutes.

Gently blot the zucchini and squash slices with paper towels to absorb remaining moisture.

In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté for 3–4 minutes until translucent. Stir in the minced garlic and cook for an additional 30 seconds.

Add the blotted squash and zucchini slices to the skillet. Sauté for 5–7 minutes until slightly tender. Remove from heat and set aside.

In a large mixing bowl, whisk together the sour cream, mayonnaise, beaten egg, black pepper, thyme, and garlic powder.

Fold in the shredded sharp cheddar, grated Parmesan, and chopped fresh parsley into the creamy mixture.

Preheat the oven to 375°F (190°C) and grease a 9×13-inch baking dish with butter or cooking spray.

Add the sautéed vegetables to the cheese mixture and gently fold together until everything is coated.

Pour the combined mixture into the greased baking dish and spread evenly.

Top with shredded mozzarella cheese and cover with aluminum foil. Bake for 20 minutes.

Remove the foil and continue baking for another 15–20 minutes until the top is golden and bubbly.

Let rest for 5–10 minutes before serving. Garnish with extra parsley if desired.