Cut the tomatoes and core them, removing all the seeds and juice.
In a medium bowl add the onions, basil, and goat cheese. Mix and season with salt and freshly ground black pepper. Add cooked orzo and combine. Stuff the tomatoes with the mixture. Drizzle olive oil over the tomatoes. Top the tomatoes with the Parmesan.
Place the tomatoes directly on the grill with the cover closed and cook for 10 minutes. The juices will begin to run. Remove from grill and place on platter.