Croissant Breakfast Casserole

Name: Croissant Breakfast Casserole
Ingredients: 1-2 tablespoons butter, (softened); 1 pound croissants, (about 5 to 7, split in half lengthwise); 1 tablespoon vegetable oil; 1 bunch scallions, (6 to 7, white and light green parts thinly sliced, greens reserved); 3/4 pound sweet Italian sausage, (casings removed); 2 teaspoons, plus 1 tablespoon for garnish fresh sage, divided, (finely chopped); 8 large eggs, (at room temperature); 3 cups whole milk; 1 cup heavy cream; 8 ounces, (2 cups) Gruyère, (grated ); 1 1/4 teaspoons salt; 1 teaspoon black pepper