Lightly coat a 9×13-inch or similarly sized casserole dish with baking spray. Scatter half the bagel pieces into the bottom of the dish. Then sprinkle over half the sausage, cream cheese and a sprinkling of cheddar. Then repeat with the rest of the bagels.
In a large bowl, combine the eggs, milk, mustard, salt, and pepper until smooth. Pour the egg mixture over the bagels, then lightly press down on the bagels so that they absorb some of the egg.
Dot the cream cheese pieces on top. Sprinkle on the remaining sausage bits, 2 tablespoons fresh spring onions and remaining Cheddar cheese. Cover the casserole tightly with plastic or foil and refrigerate for 2 hours or overnight.
When ready to bake, remove the casserole from the refrigerator and let stand at room temperature while the oven preheats. Preheat the oven to 350 degrees F. Tent the casserole with foil and bake, covered, for 20 minutes. Remove from the oven, uncover, then return to the oven and continue baking until golden at the edges and set in the center, about 15-20 additional minutes.
Let stand at room temperature for 10 minutes. Serve warm.