Heat the oven to 425°F and arrange a rack in the middle.
Line a baking sheet with aluminum foil and place the salmon skin-side down on the foil. Drizzle the oil over the salmon, rub it into an even layer with your fingers, and season generously with salt and pepper.
Bake the salmon until it’s just opaque all the way through or the center of the thickest part of the fillet registers 135°F to 140°F on an instant-read thermometer, about 10 to 17 minutes.
Using a flat spatula to get between the skin and the flesh, lift up the salmon and transfer it to a plate, leaving the skin behind. (It’s OK if the salmon falls apart a little bit.) Discard the foil and skin.
Let the salmon cool slightly at room temperature, about 5 minutes.
Refrigerate uncovered until completely chilled, at least 2 hours. (If you’re not using it right away, cover the chilled salmon and refrigerate it up to 24 hours.)
Stir together the onion and vinegar in a large, nonreactive bowl and let it sit for 15 minutes.
Using your hands, flake the chilled salmon into bite-size pieces and place them in the bowl with the onion-vinegar mixture. Add the remaining ingredients and gently stir to combine, being careful not to break up the salmon too much.
Taste and season with additional salt and pepper as needed. Serve immediately or chill for up to 4 hours