Prep the zucchini: Slice the zucchini into 1/4-inch rounds. Place in a large colander and toss with 1 teaspoon kosher salt. Let sit for 20 minutes to draw out excess moisture. Pat dry thoroughly with paper towels.

Cook the bacon: In a skillet, cook bacon until crisp. Drain on paper towels and crumble. If you like, reserve 1 tablespoon bacon fat for sautéing the aromatics.

Sauté onion and garlic: In the same skillet, heat oil (or bacon fat) over medium heat. Add onion and cook 3–4 minutes until softened. Stir in garlic for 30 seconds until fragrant. Remove from heat.

Pre-bake the zucchini: Preheat oven to 400°F (200°C). Spread zucchini rounds on two parchment-lined baking sheets. Bake for 10–12 minutes, flipping once, until just tender and releasing moisture. Pat again with paper towels if needed. Reduce oven to 375°F (190°C).

Mix the creamy base: In a bowl, whisk eggs, sour cream, and heavy cream. Add Dijon, black pepper, smoked paprika, and a pinch of salt. Stir in the sautéed onion and garlic.

Assemble the casserole: Lightly grease a 9x13-inch baking dish. Layer half the zucchini, then half the cheddar and mozzarella, and half the bacon. Pour over half the creamy mixture. Repeat with remaining zucchini, cheeses, and bacon. Sprinkle Parmesan on top and finish with chives or green onions.

Bake: Bake at 375°F (190°C) for 22–28 minutes, until the top is golden and the center is set. If you want extra browning, broil for 1–2 minutes at the end, watching closely.

Rest and serve: Let the casserole rest for 10 minutes to set. Garnish with parsley if using. Slice and serve warm.