In a large skillet, cook Italian sausage and onion over medium-high heat until the meat is no longer pink and the onion is starting to soften, about 5-7 minutes. Add the garlic; cook until fragrant, about 30 more seconds. Drain off excess grease.
Stir in tomatoes and their juices, marinara sauce, broth, Italian seasoning, and garlic salt. Stir in tortellini and bring to a boil. Reduce heat, cover, and simmer until pasta is tender, about 8-10 minutes. Stir occasionally so that the pasta doesn’t stick.
Stir in parsley and sprinkle Italian cheese blend and Parmesan cheese on top. Remove from the heat, cover, and let stand until the cheese melts, about 2 minutes. Alternatively, for a crispier browned top, place the skillet under the broiler for a few minutes (just make sure that your skillet is oven-proof!). Garnish with additional fresh parsley, if desired.