1 pound croissants (about 5 to 7), split in half lengthwise
1 tablespoon extra-virgin olive oil, more for baking dish
1 bunch scallions (6 to 7), white and light green parts thinly sliced, greens reserved
¾ pound sweet Italian sausage, casings removed
2 teaspoons finely chopped fresh sage
8 large eggs
3 cups whole milk
1 cup heavy cream
8 ounces Gruyère, grated (2 cups)
1 ¼ teaspoons kosher salt
1 teaspoon black pepper
Heat oven to 500 degrees. Spread croissants on a large baking sheet and toast, cut side up, until golden brown, 5 to 10 minutes (watch carefully to see that they do not burn). Let cool, then tear into large bite-size pieces.
In a medium skillet over medium-high heat, warm the olive oil. Add sliced scallions and sausage meat; cook, breaking up meat with a fork, until mixture is well browned, about 5 minutes. Stir in sage, and remove from heat.
In a large bowl, toss together croissants and sausage mixture. In a separate bowl, whisk together eggs, milk, cream, 1 1/2 cups cheese, salt and pepper.
Lightly oil a 9- x 13-inch baking dish. Turn croissant mixture into pan, spreading it out evenly over the bottom. Pour custard into pan, pressing croissants down gently to help absorb the liquid. Cover pan with plastic wrap and refrigerate at least 4 hours or overnight.
When you’re ready to bake the casserole, heat oven to 350 degrees. Scatter the remaining grated cheese over the top of the casserole. Transfer to oven and bake until casserole is golden brown and firm to the touch, 45 minutes. Let stand 10 minutes. Garnish with sliced scallion tops before serving.