Add tomatoes, sliced side down, onto grill. Cook for about 5-7 minutes until light grill marks begin to show.
Meanwhile, heat olive oil in a saute pan, add garlic, salt, onion powder, pepper, and rosemary. Cook for 3-5 minutes or until fragrant, stirring often ensuring that the garlic doesn't burn. Remove from heat and add to a small bowl and set aside.
Gently flip each tomato half and generously brush with garlic oil mixture and top with Parmesan cheese.
Close grill and finish cooking for another 7-10 minutes until cheese is fully melted.