How to Make It
Preheat grill to medium. Lightly oil grates and place sausage on top. Cook 5 minutes, turning once or twice. Brush vegetables with dressing; add to grill. Cook, turning periodically, until vegetables are tender and sausages are cooked through, 15 to 20 minutes. Remove to a board; let cool slightly. Chop sausage and vegetables, discarding root end of onion.
In a large pot of boiling, salted water, cook spaghetti until al dente, about 9 minutes. Drain, reserving 1 cup cooking liquid. Add pasta back to pot along with sausage, vegetables and 2 Tbsp. oil. Toss to combine, adding cooking liquid to moisten if needed. Serve with Parmesan and basil, if desired.