Preheat the oven to 350 degrees F. Grease a 9-by-13-inch baking dish thoroughly with the butter.
Heat a pan over medium-high heat. Add the sausage and cook, breaking it up with a wooden spoon, until browned, 5 to 8 minutes. Add the spinach and cook until wilted, about 1 minute. Set aside to cool.
Whisk the eggs in a large bowl with some salt and pepper. Add the cheese and stir until combined. Mix in the cooled sausage and spinach mixture.
Pour the egg mixture into the prepared baking dish and bake until the eggs are cooked through and firm, 20 to 30 minutes. Let cool slightly, about 5 minutes, then cut into squares and serve.