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1
Coat a 13-by-9-inch baking dish with butter; set aside.
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2
Cut the cream cheese into 1/2-inch cubes, place them in a single layer on a large plate, and freeze for 10 minutes (this will make them easier to sprinkle into the casserole).
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3
Place half of the bagel cubes in an even layer in the prepared dish and sprinkle with half of the cream cheese and half of the chives.
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4
Repeat with the remaining bagel cubes, cream cheese, and chives; set the dish aside.
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5
Whisk the eggs in a large bowl to break them up.
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6
Add the milk, salt, and pepper and whisk to combine.
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7
Slowly pour the mixture evenly over the bread and cream cheese and press slightly on the bread to make sure it is mostly submerged.
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8
Cover the dish with plastic wrap or aluminum foil and refrigerate overnight.
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9
When ready to bake the casserole, heat the oven to 350 degrees F and arrange a rack in the middle.
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10
Meanwhile, remove the casserole from the refrigerator and let it sit at room temperature.
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11
Uncover the casserole and bake until its puffed, golden brown, and just set in the center, about 50 minutes.
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12
Remove the dish to a wire rack.
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13
Immediately scatter the smoked salmon, capers, and onion evenly over top.
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14
Let sit for 10 minutes before serving.
