Heat the oven to 400 °F and lightly grease a 9-inch square ceramic or glass baking dish. The dish should be ready before the sausage finishes so you can assemble the casserole while everything is still warm but controlled.

Whisk the eggs, heavy cream, salt, pepper, garlic powder, and red pepper flakes in a large bowl, then fold in 1/2 cup of the shredded cheddar with a rubber spatula. The mixture should look fully blended and lightly frothy, with the cheese evenly suspended instead of sinking in clumps.

Set a large 12-inch skillet over medium-high heat and cook the sausage without added oil for 5 to 7 minutes, breaking it into small pieces as it browns. Stop when you see browned bits and the crumbles still look juicy, because that’s where the best flavor develops.

Turn off the heat and stir in the spinach leaves while the pan is still hot. The leaves should soften only slightly and stay bright green rather than collapse into a dark, wet pile.

Spread the sausage and spinach mixture evenly in the prepared baking dish. An even layer helps the eggs set at the same pace across the whole pan.

Whisk the egg mixture once more, then pour it over the sausage mixture and gently nudge the filling if needed so it distributes evenly. You want sausage and spinach tucked throughout the custard, not packed into one corner.

Scatter the remaining 1/2 cup cheddar over the top. The surface should look evenly covered so you get a consistent golden finish.

Bake for about 25 minutes, until the cheese is golden and the eggs are set with a slight jiggle in the very center. You’ll notice a gentle toasted-cheese aroma, and the middle should wobble a little rather than slosh.

Let the casserole rest for 10 minutes before slicing it into nine squares. That short rest allows the egg structure to settle so the pieces cut cleanly instead of running at the edges.