Spray an 8" x 8" baking dish with nonstick cooking spray or brush it with oil.
Place bagel slices in the dish. Add any mix-ins and toss to combine.
In a mixing bowl, whisk together eggs, milk, cream, salt, black pepper and half the sliced chives. Pour liquids over the bagel slices. Press down gently on the bagels to fully soak in liquids.
Refrigerate casserole, covered, for at least 1 hour or overnight.
When ready to bake, preheat oven to 350 degrees F. Bake casserole until eggs are set, about 1 hour. If the top of the casserole starts to turn dark brown, cover the top loosely with foil until it finishes baking.
While casserole bakes, combine cream cheese and scallions. Beat with a mixer (or use a stand mixer) until light and fluffy.
Let cool for 5 minutes before slicing.
Serve casserole with lox, capers and whipped scallion cream cheese on the side.