Spray a 9x13-inch dish; arrange bagel pieces evenly.
Distribute cream cheese, sausage, cheddar, onion, and scallions evenly over bagels.
Whisk eggs, milk, 2 tablespoons seasoning, and pepper in a large bowl.
Pour evenly over filling; press lightly to soak bagels.
Top with remaining 1 tablespoon seasoning; cover and refrigerate at least 4 hours or overnight.
Heat oven to 350°F; bake foil-covered 30 minutes, then uncovered 20 minutes until golden and center set. Rest 10 minutes. To freeze, cool completely, wrap tightly in plastic and foil, and freeze up to 3 months. Thaw refrigerated overnight; reheat at 325°F for 30-35 minutes.