How to Make It
In a bowl, gently combine the sirloin and truffle and shape into 4 thick burgers. Set the burgers on a plate, cover with plastic wrap and let stand at room temperature for 30 minutes.
In a large heavy skillet, heat the olive oil until shimmering. Season the burgers with salt and pepper and cook over high heat until browned, about 3 minutes per side for medium rare. Set the burgers on the toasted English muffins.
Toss the watercress with the walnut oil and lemon juice. Season with salt and pepper and serve with the burgers.