Heat broiler, with rack in top position. In a small saucepan, heat oil over medium-high. Add shallot and cook, stirring often until softened, 3 minutes. Add vinegar and cook until slightly evaporated, 1 minute. Add mustard and brown sugar; stir until warm and combined, 1 minute. Season with salt and pepper and remove from heat. (To store, refrigerate cooled glaze in an airtight container, up to 1 day.)
Place salmon fillets on a foil-lined rimmed baking sheet and season with salt and pepper. Transfer 1/2 cup glaze to a small dish and brush on top of salmon. Broil salmon until glaze is bubbling and fish is opaque throughout, 5 to 10 minutes, depending on thickness; brush remaining glaze over fillets. Serve salmon along with watercress and lemon wedges.