Heat a large skillet over medium heat.  Add sausage and cook, using a wooden spoon to break it up into small pieces until cooked through, about 10 minutes. Add the diced onion to the skillet and cook until softened, about 2-3 minutes.

Spray or butter your baking dish.

Slice the bagels into 1-inch cubes and place them into the prepared baking dish.

Whisk together the eggs, salt, pepper, milk, cream, Dijon mustard, spinach, and shredded cheese until combined.

Cut the cream cheese into 1/4 inch pieces and scatter over the sliced bagels in the prepared baking dish.

Combine the sausage and onions with the eggs and stir until combined.   Pour the egg sausage mixture over the bagels and stir gently to combine, ensuring each bagel is coated with the custard.  Sprinkle the everything bagel seasoning over the casserole. Cover with plastic and refrigerate for 2 hours or overnight.

Bake in a preheated 350° F oven for 40 - 45 minutes until golden brown and the eggs are cooked through.