Step 1

Prepare grill for direct medium-high grilling.

Step 2

In a small bowl combine vinegar, olive oil, mustard, salt, and pepper.

Step 3

Put mushrooms on a plate top-side-up. Brush tops lightly with the vinegar mixture. Flip mushrooms over. Brush remaining marinade liberally on the gills and let it soak in for a minute.

Step 4

Grill mushrooms top-side-down until grill marks are dark, 3-4 minutes. Flip and grill for another 3-4 minutes. Remove from grill and let rest for 5 minutes.

Step 5

Slice and serve.