Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish with olive oil or cooking spray.

Carefully unroll the puff pastry sheet (or crescent roll dough) and gently press it into the prepared baking dish, allowing it to extend slightly up the sides to form a crust. Prick the bottom of the pastry with a fork a few times to prevent excessive puffing during baking.

In a large non-stick skillet, cook the ground breakfast sausage over medium heat, breaking it apart with a spatula, until it's browned and cooked through, about 5-7 minutes. Drain any excess grease.

Add the diced yellow onion and bell pepper to the skillet with the cooked sausage. Sauté until the vegetables have softened, about 5 minutes. If using, add the chopped fresh spinach and cook until wilted, about 1-2 minutes. Remove the skillet from heat and set the mixture aside to cool slightly.

In a large mixing bowl, whisk together the large eggs, milk (or half-and-half), salt, black pepper, and red pepper flakes (if using) until everything is well combined and slightly frothy.

Stir the shredded cheddar cheese into the egg mixture.

Add the slightly cooled sausage and vegetable mixture to the egg and cheese mixture, stirring until all ingredients are evenly distributed.

Pour the entire filling mixture into the puff pastry-lined baking dish, spreading it out evenly.

Bake for 30-35 minutes, or until the egg mixture is set in the center (a knife inserted into the center should come out clean) and the pastry crust is golden brown and flaky.

Remove the baking dish from the oven and let it rest for 5-10 minutes before slicing into 8 equal pieces. Garnish with fresh chopped parsley if desired. Serve warm.