The day before serving, prepare casserole. Start by preheating the oven to 425°. Cut croissants into large 2-inch cubes. Toast on a baking sheet between 5-10 minutes until golden brown and crispy. Remove from oven and let cool.
In a large skillet, heat olive oil and brown sausage, breaking up sausage as it cooks. Once sausage is cooked through, add crushed red pepper if using and stir in fresh sage.
In a large bowl, whisk together eggs, milk, cheddar cheese, parmesan cheese, onion powder, garlic powder, salt, and pepper.
Spray casserole dish with nonstick cooking spray. Combine toasted croissants and sausage in baking dish. Then pour egg and cheese mixture evenly over croissants and sausage, pressing down lightly to allow croissants to absorb eggs. Cover tightly and refrigerate overnight.
Before baking, let casserole sit at room temperature for one hour. Preheat oven to 350° and bake uncovered for 45 minutes until casserole is golden brown. Top with fresh chives.