Place the sliced zucchini in a colander and toss with 1/2 teaspoon of the salt. Let it sit for 15 minutes to draw out excess moisture, then pat dry well.

Preheat the oven to 375°F and lightly grease an 8x8-inch baking dish.

Heat the olive oil in a skillet over medium heat. Add the diced onion and cook for 4 to 5 minutes until softened.

Add the minced garlic and cook for 30 seconds, just until fragrant.

In a large bowl, whisk together the eggs, sour cream, heavy cream, cheddar cheese, 1/4 cup Parmesan, remaining 1/2 teaspoon salt, black pepper, thyme, and smoked paprika.

Fold the zucchini and onion mixture into the egg mixture until evenly combined.

Spread the mixture evenly in the prepared baking dish.

In a small bowl, mix the panko breadcrumbs, remaining 1/4 cup Parmesan, and melted butter. Sprinkle evenly over the casserole.

Bake for 35 to 40 minutes, until the center is set and the top is golden brown. Broil for 1 to 2 minutes at the end if needed for extra color, watching carefully.

Let the casserole rest for 10 minutes before serving. Garnish with chopped parsley if desired.