How to Make It
Mix chili sauce, soy sauce, and vinegar in a bowl.
Heat 1 tablespoon oil in a heavy pan over medium-high. Season salmon with salt and pepper. Cook until browned on 1 side, about 3 minutes. Flip and cook, brushing with soy mixture often, until just cooked through, about 3 minutes.
Toss watercress, radishes, lemon juice, and remaining 1 tablespoon oil in a medium bowl.
Spread mayonnaise on top and bottom halves of buns. Build sandwiches with salmon and watercress mixture. Serve sandwiches with sweet potato chips.