2) While the pasta is cooking: Heat 4 tablespoons of the oil, over medium-high heat, in a large dutch oven or heavy bottomed saucepan. Add the onion, season with salt and pepper, and cook until soft, about 5 to 7 minutes. Add the garlic and cook for one minute longer. Add the marsala wine and grape tomatoes, and cook for one minute, scraping the brown bits that cling to the bottom of the pan with a wooden spoon. Add the clams and mussels to the pan. Cover the pan with a tight-fitting lid and cook until all the shellfish have opened, about 5 to 8 minutes. Discard any unopened shellfish.
2) Using tongs, remove the shellfish from the pan and reserve. Drain the pasta and transfer to a large serving bowl. Pour the shellfish cooking liquid over the fregola. Add the remaining olive oil and 4 tablespoons of the parsley and toss. Place the reserved shellfish on top of the fregola and garnish with the remaining parsley.