Preheat the oven to 400 degrees F. Line a baking sheet with foil and coat with cooking spray.
Add the potatoes to a medium saucepan and cover with cold water. Bring to a boil over medium heat and cook until the potatoes are fork tender, about 15 minutes. Reserve 1/2 cup of the starchy water and drain the potatoes. Return the potatoes to the pan, add the cream cheese, scallions, the reserved starchy water and 2 teaspoons salt. Mash with a potato masher or fork.
Meanwhile, mix together the bagel chips, everything bagel seasoning and 1 tablespoon oil in a small bowl.
Press the bagel chip mixture onto the top of each salmon fillet. Transfer the fillets to the prepared baking sheet, skin side-down. Bake until the salmon is cooked through and the top is golden brown, 10 to 12 minutes.
Arrange the tomato slices and onions on 4 plates and drizzle with the vinegar and remaining tablespoon of oil. Sprinkle with capers, if using, and season with salt and pepper. Divide the salmon and mashed potatoes among the plates.
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