Heat oven to 375°F. Lightly butter a 2-quart baking dish.
Toss sliced zucchini with 1 teaspoon salt. Let stand 10 minutes to draw out moisture, then pat very dry.
Warm olive oil in a skillet over medium heat. Soften onion 4–5 minutes; stir in garlic for 30 seconds. Cool slightly.
Whisk eggs, sour cream, milk, Italian seasoning, pepper, and remaining 1/4 teaspoon salt in a large bowl.
Stir in cheddar, mozzarella, and half the Parmesan. Fold in zucchini and the onion mixture until coated.
Spread mixture in the baking dish. Combine breadcrumbs, melted butter, and remaining Parmesan; sprinkle evenly on top.
Bake 25–30 minutes until bubbling and golden. Rest 10 minutes before serving. Garnish with parsley.
Notes
Variation: Swap in pepper jack for a mild kick, or add 1 cup cooked shredded chicken to make it a hearty main. Storage: Refrigerate leftovers up to 3 days; reheat covered at 350°F until warm and crisp the topping under the broiler for 1–2 minutes if needed.This recipe is an original creation inspired by classic Cheesy Zucchini Casserole Recipe flavors. All ingredient ratios and instructions are independently developed.