- Heat broiler, with rack on top shelf. Using a small saucepan, heat oil over medium high. Add shallot and cook, stirring often to avoid browning until softened, about 3 minutes. Add vinegar and cook until slightly evaporated, 1 minute. Add mustard, maple syrup, and brown sugar; stir until warm and combined, 1 minute. Season with salt and pepper and remove from heat.
- Place fish on foil-lined rimmed baking sheet (make sure the foil is liberally sprayed with pan spray) 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 fish.
- Serve with watercress and lemon wedges.
Wine pairing: Chardonnay, Sauvignon Blanc, or Malbec