Grilled Portobello Mushrooms

Every year we try something new on the grill one year it was grilled pineapple, another year it was pork lion on a cedar plank.
I have a recipe for grilled portobello mushrooms that we tried tonight; the recipe follows. They were actually rather good we will make them again I am sure

Recipe:
4-6 Portobello Mushrooms
Balsamic Vinegar
Salt and ground black pepper
Dinner Rolls
Sliced tomatoes
Lettuce

Clean mushrooms with a damp towel lay them plate. Brush caps with balsamic vinegar and sprinkle with salt and pepper. Heat grill to medium and spray with non-stick cooking spray. Place mushroom on the grill gill side down and cook for 4 minutes. Turn and continue to grill until mushrooms are almost cooked through, about 4 minutes.
Remove and drain mushrooms on paper towel, gill side down. Serve on roll & top with onion and tomato.