How to Make It
Stir the saffron into the mayonnaise; season with salt and pepper. Toss the watercress, fennel, shallot and lemon juice; season.
Melt the butter in the oil in a skillet. Season the fish with salt and pepper; cook over high heat for 3 minutes per side. Spread the bread with the saffron mayonnaise; sprinkle with paprika. Fill the sandwiches with the fish, peppers, tomato and watercress salad, and serve.