Remove sausage from casings. Cook in a large skillet over medium-high heat, breaking it up.
Add chopped onion and cook until softened, about 5-7 minutes.
Stir in minced garlic, red pepper flakes (if using), oregano, salt, and pepper. Cook until fragrant, about 1 minute.
Drain excess fat from the skillet.
Let the sausage mixture cool slightly for 10-15 minutes.
Whisk egg in a small bowl for egg wash.
Slice croissants horizontally, leaving a hinge.
Divide sausage mixture among croissants and top with shredded mozzarella cheese.
Brush croissants with egg wash.
Preheat oven to 375°F (190°C). Place croissants on a baking sheet lined with parchment paper.
Bake for 12-15 minutes, or until golden brown and cheese is melted and bubbly.
Let croissants cool for a few minutes before serving.