1. Preheat oven to 375°. Sauté onion in hot oil in a large skillet over medium-high heat 5 to 6 minutes or until tender. Add chopped tomato, garlic, and 1 1/2 cups small tomatoes, and sauté 10 minutes or until tomatoes are softened. Remove from heat, and stir in vinegar and next 4 ingredients.
2. Place remaining small tomatoes in a 13- x 9-inch baking dish. Spoon onion mixture over tomatoes, and gently toss to coat. Bake at 375° for 10 minutes.
3. Meanwhile, stir together flour and next 2 ingredients in a large bowl. Cut butter into flour with a pastry blender until mixture resembles small peas; cover and chill 10 minutes. Stir cheese and next 2 ingredients into cold flour mixture. Add buttermilk, stirring just until dry ingredients are moistened. Dollop mixture by 1/2 cupfuls onto tomato mixture. (Do not spread.)
4. Bake at 375° for 30 to 35 minutes or until golden brown. Cool on a wire rack 30 minutes before serving.