Bring a large pot of salted water to a boil.
Heat oil in medium-size nonstick skillet over medium heat. Cut sausage in 1-inch pieces. Add to skillet and cook 5 minutes; add onion and cook 5 minutes more or until onion is softened and lightly colored.
Meanwhile, cook spaghetti in boiling water until al dente, about 8 minutes, tender but still firm. Drain thoroughly.
While spaghetti cooks, add half-and-half and tomatoes to sausage in skillet and warm through over medium-low heat for a few minutes. Drain pasta; return to cooking pot. Add sausage sauce and basil and toss with pasta to combine; cook 1 minute. Sprinkle Parmesan on top; toss and serve.
