Grilled Portobello and Boursin Sandwich

Procedure

1. With a wire whip, mix together the oil, vinegar, mustard, and salt to make a vinaigrette.
2. Brush the mushroom cap with about two-thirds of the vinaigrette. Reserve the rest of the vinaigrette for step 6.
3. Place the mushroom cap on a grill or broiler grid, stem side down. Grill until about half cooked.
4. Turn the mushroom over