1) Chop the shrimp and then wrap it in a thin plastic sheet. Beat with a mallet till the meat is crushed.
2) Preheat the grill for barbecuing.
3) Put the shrimp in a bowl and add the seasoning (1 teaspoon salt and 1 teaspoon pepper). Mix well and set aside.
4) Mix together the onion, tomato, salt and pepper. Set aside.
5) Mix together the butter and garlic and melt the mixture in the microwave for a few seconds.
6) Brush the insides of the mushroom halves with the garlic and butter mixture. Stuff each half with some shrimp mixture and then some tomato and onion mixutre.
7) Top each stuffed mushroom half with both the cheeses.
8) Put the stuffed mushroom caps in foil-lined cooking boxes. Place the boxes on the hot grill and let the mushrooms cook for about 5-8 minutes, till the ingredients are cooked and the cheese has melted.
9) Serve the stuffed mushrooms while they are still hot.