Step 1: Slice the zucchini and place in a colander. Sprinkle with salt and let sit for 20 minutes to draw out moisture.
Step 2: Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking dish.
Step 3: Heat olive oil in a skillet over medium heat. Sauté the onions and garlic until soft and translucent, then set aside to cool slightly.
Step 4: Rinse the zucchini briefly and pat thoroughly dry with paper towels. Ensure all excess water is removed.
Step 5: In a large bowl, whisk together the sour cream, egg, 1 cup of cheddar cheese, and black pepper.
Step 6: Gently fold the dried zucchini slices and the sautéed onion mixture into the sour cream base.
Step 7: Pour the mixture into the prepared baking dish. Spread it out evenly.
Step 8: Mix the Panko breadcrumbs, melted butter, and the remaining 1/2 cup of cheddar and parmesan in a small bowl.
Step 9: Sprinkle the breadcrumb mixture evenly over the top of the zucchini.
Step 10: Bake for 30-35 minutes until the casserole is bubbly and the top is golden brown. Let rest for 10 minutes before serving.