Preheat oven to 375°F (190°C) and slice croissants in half, then cut into cubes. Grease a casserole dish and layer croissant cubes.

In a skillet, crumble and brown sausage over medium heat for 5-7 minutes. Add shallots and garlic and sauté for 2-3 minutes. Cool and drain excess grease.

In a bowl, whisk together eggs, milk, half-and-half, salt, pepper, and Dijon mustard. Fold in two-thirds of the cheese.

Spread sausage mixture over croissants in the dish. Pour egg mixture over the top and gently toss to combine.

Sprinkle remaining cheese on top, cover with foil, and bake for 35 minutes. Remove foil and bake for an additional 5 minutes until golden.

Let cool for 5 minutes, garnish with green onions, cut into squares, and serve warm.