This is one of those great summer meals that’s elegant enough for company yet quick and easy to make. Peppery watercress and mustard bring a wonderful brightness to the salmon’s rich flavor.
At least 30 minutes, or up to a day, before serving, prepare sauce: Stir together hot butter, watercress, mustard, and shallots. Season with salt and pepper to taste.
Meanwhile, set up your grill for direct high heat. Brush clean grill grate with oil; then lay salmon, skin side up, on grate. Close lid and cook 5 minutes; then gently turn salmon skin side down and dot with about half the sauce. Close lid and cook 3–5 minutes longer, until just opaque (cut to check).
Transfer to plates and garnish with watercress sprigs. Serve with remaining sauce, warmed.