1. Preheat oven to 375°F (190°C) and grease a casserole dish well.
2. Heat olive oil in a large skillet over medium heat. Add diced onion and minced garlic and sauté for 3 to 4 minutes until softened and fragrant.
3. Add the sliced zucchini and cook for a full 5 minutes, stirring occasionally, until slightly softened and the excess moisture has cooked off. Season with salt, pepper, Italian seasoning, and garlic powder directly in the skillet.
4. In a large bowl, whisk the sour cream and beaten eggs until smooth. Stir in half the mozzarella and all the Parmesan.
5. Add the sautéed zucchini mixture to the bowl and fold together gently until every piece is coated. Taste and adjust seasoning — this is the last easy window before baking.
6. Transfer to the prepared casserole dish and spread evenly. Scatter remaining mozzarella over the top, then sprinkle breadcrumbs evenly over the cheese.
7. Bake uncovered for 25 to 30 minutes until golden, bubbling at the edges, and set through the center when shaken gently.
8. Rest for 5 minutes before serving — it slices much more cleanly with a brief rest (if you can wait that long).

Nutrition Information (Per Serving):

• Calories: 280
• Carbohydrates: 14g
• Protein: 15g
• Fat: 19g
• Fiber: 2g
• Sodium: 420mg
• Key vitamins/minerals: Calcium (30% DV), Vitamin C (25% DV), Vitamin A (20% DV), Potassium (18% DV) Note: Zucchini is exceptionally low in calories while contributing meaningful amounts of vitamin C, potassium, and antioxidants, making this rich, satisfying casserole more nutritionally generous than its indulgent character suggests.

Notes:

• Pre-cooking the zucchini in the skillet for the full 5 minutes is the single most important step — skip it and excess moisture turns the casserole watery no matter how carefully everything else is done.
• Season the zucchini directly in the skillet rather than waiting until the bowl — the vegetables absorb flavors much more effectively while still hot.
• Taste the mixture before transferring to the baking dish — this is the last opportunity to adjust salt before the egg sets everything in place.
• Whole-milk mozzarella shredded from a block melts dramatically better than pre-shredded bags — it’s worth the extra two minutes of grating.

Storage Tips:

• Refrigerate in an airtight container for up to 4 days — actually slices more cleanly cold than straight from the oven.
• Reheat covered loosely with foil in a 350°F oven for 12 minutes for best results, or microwave individual portions covered for 90 seconds.
• Freeze before baking for up to 2 months — thaw overnight in the refrigerator and bake as directed adding 10 extra minutes.
• The breadcrumb topping can be added fresh before baking from frozen rather than assembled ahead for the crispiest possible top.

Serving Suggestions:

• As a satisfying vegetarian main course alongside a crisp green salad with sharp lemon vinaigrette
• As a hearty side dish next to roasted chicken or grilled fish for a complete summer dinner
• With warm crusty bread for scooping up the creamy, cheesy filling from the edges of the dish
• Finished with a scattering of fresh basil leaves torn over the top right before serving for a bright, aromatic finish

Mix It Up (Recipe Variations):

• Summer Garden Version: Add diced yellow squash and halved cherry tomatoes to the skillet with the zucchini for a colorful, vibrant seasonal casserole.
• Spicy Cheesy Zucchini Casserole: Add red pepper flakes and a diced jalapeño with the aromatics for a warming heat that plays beautifully against the creamy base.
• Cheesy Zucchini and Corn Casserole: Stir a cup of fresh or frozen corn kernels into the filling for a sweet, summery twist that works beautifully with the Parmesan.
• Heartier Main Course Version: Stir half a cup of cooked crumbled Italian sausage into the filling before baking to turn this into a genuinely filling one-dish dinner.

What Makes This Recipe Special:

Pre-cooking the zucchini in olive oil with the aromatics for a full 5 minutes before assembling draws out the excess moisture that would otherwise pool at the bottom of the baking dish and makes the difference between a watery disappointment and a casserole that slices cleanly and holds its shape beautifully. Whisking the sour cream and eggs into a smooth base before folding in the zucchini creates a binding layer that sets during baking into a rich, custardy interior that gives this cheesy zucchini casserole genuine substance and satisfying body. The two-layer cheese approach — mozzarella folded through the filling for creaminess and mozzarella scattered on top for that golden, bubbling crown — means every single bite from the center of the dish to the crispy breadcrumb edges delivers the full cheesy, satisfying experience this casserole promises from the moment it comes out of the oven.