Preheat oven to 350°F (175°C) and butter a 9x13 inch baking dish. In a skillet over medium-high, cook sausage 6-8 minutes until browned. Add onion and pepper, cook 4 minutes. Add garlic and spinach and cook until wilted and dry, 2 minutes.

Whisk eggs, heavy cream, salt, pepper, and red pepper flakes in a large bowl until pale and frothy, about 30 seconds.

Spread sausage mixture in the dish, top with half the cheese, pour eggs over everything, then sprinkle remaining cheese on top.

Bake until center is set with a slight jiggle and edges are golden, 35-40 minutes. A knife inserted near the center should come out clean.

Let rest 5 minutes, then slice into 8 squares and serve warm.