In a 2-qt saucepan over medium heat, melt 1 tbsp butter. Add onions and cook until soft - about 4 to 5 minutes. Add garlic and cook 2 minutes. Add rice and stir to coat for 1 minute. Stirring constantly, add 1/2 cup of simmering chicken broth. As rice absorbs the liquid, continue to add broth, 1/4 cup at a time, until you've added 3 cups, about 15 minutes. Reduce heat to low and cook rice for an additional 20 to 30 minutes until tender, stirring occasionally. If rice starts to dry out, stir in more broth.
When rice is done, remove pan from heat. Let stand for 5 minutes. Stir in remaining butter and cheese. Fold in all but 3 tablespoons tomatoes. Season with sat and pepper to taste. Divide risotto evenly among 6 bowls. Garnish each bowl with remaining tomatoes and basil.