Meanwhile, combine garlic, vinegar, salt, and pepper in a bowl. Slowly whisk in oil; brush over mushrooms and let stand for 20 minutes. Remove mushrooms from bowl and discard marinade.
Set mushrooms on grill rack and grill until tender, 4 to 6 minutes per side.
Meanwhile, combine blue cheese, mayonnaise, cream and Worcestershire sauce in a bowl; mix well, mashing chunks of cheese with the back of a spoon or fork.
To assemble, place half a bun on each of the 4 plates; top with lettuce leaf, a mushroom cap, a tomato slice, and optional onion slice. Spoon 2 tablespoons of blue cheese sauce over each and top with the other half bun.
Serve warm or at room temperature.
Enjoy!!!
Note: Portobello mushrooms can also be cooked under broiler or on a grill pan.