Bring a large pot of salted water to a boil. Add pasta and cook for 12 minutes, or until al dente. Drain thoroughly and return to pot. Toss with 1 Tbsp. oil to prevent pasta from sticking.
While pasta is cooking, heat 2 Tbsp. oil in a large skillet over medium-high heat. Remove sausage from casings and add to skillet, breaking meat apart with a fork or spatula. Season with 1/2 tsp. salt and 1/8 tsp. pepper and cook until nicely browned and cooked through, 5 to 7 minutes. Transfer to a plate lined with paper towels. Wipe any excess oil from pan.
In same large skillet, heat remaining 2 Tbsp. oil and garlic over medium-low heat until garlic just turns golden and oil is infused. Discard garlic. Add tomatoes, onion and remaining 1/2 tsp. salt and 1/8 tsp. pepper. Cook over medium heat until just heated through, about 2 minutes. Add tomatoes and sausage to pasta and toss to combine. Serve garnished with parsley sprigs, if desired.