1. In a small bowl, combine mustard, olive oil, vinegar, marjoram, and black pepper. Mix well. Pour mixture into a large flat baking pan.
2. Add flank steak and quartered onions to the pan; turn to coat thoroughly with the marinade. Set aside to marinate for at least 40 minutes, at room temperature. If preparing in advance, cover the pan with plastic wrap and refrigerate. Do not marinate for more than 24 hours. Remove from refrigerator at least 20 minutes before grilling, to make grilling more efficient.
3. At least 20 minutes before cooking the steaks, prepare the grill. When the grill is hot, spray or brush with vegetable oil.
4. Remove steak and onions from the marinade and place on the grill. Season the steak and onions with salt. Handle the onions carefully to keep the quarters intact, if possible. Turn onions after about 3 minutes, continuing to turn until they look slightly opaque. Grill steak for about 4 to 5 minutes, until well marked, then turn and grill another 4 minutes. Be careful not to overcook. Flank steak is best served medium; well-done flank steak will be tough.
5. While steak is grilling, divide watercress among four plates.
6. Remove steak and onions from grill and let rest on a cutting surface for 2 minutes.
7. Slice steak diagonally, with knife at a slight angle. Lay slices on top of the watercress. Top with onions and crumbled goat cheese and serve.
Source: Hannaford fresh Magazine, July - August 2006