For the Meyer lemon-honey mustard: Whisk together in a small bowl the mustard, honey, orange juice, lemon zest, lemon juice and season with salt and pepper. Cover and refrigerate for at least 30 minutes and up to 24 hours to allow the flavors to meld.
For the burgers: Preheat the grill to high. Form the meat into four 6-ounce burgers. If you are not going to cook right away, keep the burgers cold in the refrigerator. Sprinkle the burgers on both sides with salt and pepper. Grill until cooked through, 3 to 4 minutes on each side. During the last minutes of cooking, add 2 slices of cheese to each burger, cover the grill and let the cheese soften slightly, about 1 minute. Put the burgers on the buns, drizzle with some of the Meyer lemon-honey mustard and top with a few sprigs of watercress. Serve with extra mustard on the side.
If you are unable to find Meyer lemons, mix 1/2 teaspoon fresh orange juice and 1/2 teaspoon fresh lemon juice.