Turn the oven up to 500 degrees. Toast the croissants, cut side up, on a large baking sheet in the oven until golden brown, which should take between 5 and 10 minutes (watch carefully to see that they do not burn). Once it has cooled, cut it into large pieces that are easily swallowed.
Olive oil should be warmed up in a skillet of medium size over medium-high heat. Add the cut scallions and the sausage meat, and then sauté the combination while breaking up the meat with a fork until it is nicely browned, which should take about 5 minutes. After stirring in the sage, turn off the heat.
Combine the croissants and sausage mixture in a large bowl and toss to combine. Eggs, milk, cream, 1 1/2 cups of cheese, salt, and pepper should be mixed together in a separate mixing dish using a whisk.
Prepare a baking dish that is 9 by 13 inches and lightly grease it. After pouring the croissant mixture into the pan, evenly distribute it throughout the base of the pan. After the custard has been poured into the pan, gently press the croissants down to help them absorb the liquid. Place the pan in the refrigerator for at least four hours, or even overnight, and cover it with plastic wrap.
Preheat the oven to 350 degrees Fahrenheit when you are ready to prepare the casserole. Distribute the remaining grated cheese across the surface of the dish in an even layer. After transferring the casserole to the oven, bake it for 45 minutes, or until it is golden brown and firm when touched. Let stand for 10 minutes. Before serving, garnish with the sliced tops of the scallions.