Step 1 Heat the oven to 350 degrees.
Step 2 Cut the tomatoes in half lengthwise. Set aside. Remove the papery outer skin of the garlic and chop the cloves into tiny pieces. Set aside. Cut the basil leaves into small pieces and set aside.
Step 3Place the tomatoes, cut side up, in a 13x9-inch baking dish. Salt and pepper them lightly.
Step 4Mix together the garlic, 1/3 cup of the parsley and 2 tablespoons of the olive oil in a small bowl. Place the garlic mixture on the tomatoes and drizzle 2 more tablespoons of the oil over all.
Step 5Bake uncovered until the tomatoes are mushy, about 45 to 55 minutes. About 20 minutes before the tomatoes are done, put a large pot of salted water on for the spaghetti. When the water is boiling, add the spaghetti, stir, and after about 12 minutes, start testing every minute or so until the spaghetti is tender, but firm. Drain.
Step 6Mix together the remaining parsley and olive oil, the butter and the basil in a large bowl. Add the tomatoes and spaghetti. Toss to mix well, and serve.