Firstly, preheat the oven to 500 F. Place croissants on a dish and toast until golden brown, about 5-10 minutes (careful not to let them burn). After it has cooled down, then tear it into bite-size pieces.
Next, heat olive oil in a large skillet over medium-high heat. Add the sliced scallions and sausage meat. Stir frequently with a fork as the mixture browns, taking about five minutes. When done, stir in the sage before removing it from the heat.
In a bowl, combine the croissants and sausage blend. Once you’ve done it, take another mixing bowl and whisk together eggs, milk, cream, 1 1/2 cups of cheese, salt and pepper.
Oil a 9- x 13-inch baking dish with a small amount of oil. Place the croissant mixture in the dish, and spread it evenly throughout. Pour the custard into the croissant mixture and press down gently to help them absorb the liquid. Now, cover the pan with plastic wrap and store it in the refrigerator for at least 4 hours or overnight.
Once the casserole is ready for baking, preheat your oven to 350 degrees. Sprinkle the remaining shredded cheese over the top of the dish and place it in the oven. Bake until golden brown and firm to the touch, approximately 45 minutes. Let stand for 10 minutes before garnishing with sliced scallion tops and serving.