Preheat and prep. Heat your oven to 350°F (175°C).
Lightly grease a 9x9-inch baking dish or an 8x11-inch casserole with cooking spray or butter.
Cook the bacon. In a skillet over medium heat, cook bacon until crisp. Transfer to a paper towel–lined plate and crumble when cool. Reserve 1 tablespoon of drippings for the onions, or use olive oil.
Soften the onion. Add onion to the skillet with drippings or oil. Sauté 3–4 minutes until translucent.
If using spinach, stir it in for the last 30 seconds just to wilt. Remove from heat and let cool slightly.
Whisk the eggs. In a large bowl, whisk eggs until smooth and slightly frothy. Stir in cottage cheese, shredded cheese, garlic powder, paprika, salt, pepper, and chives.
Fold in the mix-ins. Add the cooled onion (and spinach, if using) along with the crumbled bacon.
Stir gently to distribute everything evenly.
Pour and smooth. Transfer the mixture to the prepared baking dish. Smooth the top with a spatula so it bakes evenly.
Bake. Bake for 28–35 minutes, or until the center is just set and the edges are lightly golden. A slight wobble is okay—it will continue to set as it rests.
Rest and slice. Let the bake cool for 10 minutes. Slice into squares and garnish with extra chives.
Serve warm.