Preheat the oven to 375°F and lightly grease a 9x13 inch baking dish.
Cook the sausage in a skillet over medium heat until fully browned. Break the sausage apart as it cooks. Drain the excess grease and set the sausage aside.
Sauté the veggies in the same skillet using the residual sausage grease. Cook the bell pepper and onion for 3 to 4 minutes until slightly softened, then stir in the spinach until wilted. Remove from heat and set the veggies aside.
Prepare the crescent base by unrolling the dough and pressing it evenly into the bottom of the baking dish. Pinch seams together to form a solid layer.
Layer the filling by spreading the cooked sausage evenly over the crescent dough, followed by the sautéed vegetables and shredded cheese.
Make the custard by whisking together the eggs, milk, garlic powder, salt, black pepper, and red pepper flakes until fully combined.
Pour the custard mixture evenly over the casserole, allowing it to soak into the layers.
Bake for 35 minutes, or until the center is set and the top is lightly golden. The temperature should be 165°F for a perfect custard texture.
Rest for 10 to 15 minutes before serving to allow the custard to fully set, then slice and serve.