Spaghetti with Grilled Zucchini Sauce

Serves 4 12 ounces thin spaghetti 8 Italian plum tomatoes, halved 2 medium-large zucchini, cut into rounds on a slight diagonal 1 large red onion, cut into thick slabs 1 small whole garlic head 1/4 cup olive oil, divided 2 teaspoons dried oregano Salt and ground black pepper 1/4 cup pitted kalamata olives 1/2 cup...