Croissant Breakfast Casserole

Ingredients (12)

• 8 croissants, chopped into 3-in. pieces
• 1/2 pound bulk pork sausage
• 1/2 medium onion, finely chopped
• 8 large eggs
• 1-1/2 cups whole milk
• 1 cup half-and-half cream
• 1 tablespoon Dijon mustard
• 1 teaspoon poultry seasoning
• 1 teaspoon salt
• 1/2 teaspoon pepper
• 1-1/2 cups shredded Gruyere cheese, divided
• 2 tablespoons minced chives