Preheat your oven to 375°F (190°C) and thoroughly grease a 9x13-inch baking dish with the 1 tablespoon of unsalted butter.
Heat 2 tablespoons of olive oil in a large skillet over medium heat, then add the finely chopped onion and diced bell pepper, cooking for 5-7 minutes until softened.
Add the roughly chopped spinach to the skillet and cook for an additional 2-3 minutes, just until the spinach has wilted, then remove the skillet from the heat.
In a large mixing bowl, whisk together the 12 beaten eggs, 1 cup of whole milk, 1/2 cup of heavy cream, 1 teaspoon of kosher salt, 1/2 teaspoon of black pepper, and 1/2 teaspoon of garlic powder until the mixture is smooth and uniform.
Spread the cooked vegetable mixture evenly across the bottom of the prepared baking dish, then sprinkle the 1 cup of cheddar cheese and 1/2 cup of Parmesan cheese on top.
Pour the egg mixture over the vegetables and cheese, gently shaking the dish to ensure the liquid settles into an even layer.
Bake for 35-45 minutes, or until the edges are golden brown and the center is fully set, with no liquid jiggle when the dish is gently shaken.
Let the bake rest for 10 minutes on a wire cooling rack before slicing and serving, which allows it to firm up for cleaner slices.
