Heat olive oil in a large deep skillet or Dutch oven over medium heat.
Add the Italian sausage, breaking it up with a spoon, and cook until browned and cooked through, about 5-7 minutes.
Add the chopped onion and cook until softened, about 3 minutes.
Stir in the minced garlic and cook for 1 minute until fragrant.
Pour in the diced tomatoes with their juices, chicken broth, and heavy cream. Stir to combine.
Add the dried Italian seasoning, red pepper flakes, salt, and black pepper. Bring the mixture to a simmer.
Add the cheese tortellini to the pot, stirring gently to submerge them in the sauce.
Cover and cook for 7-9 minutes, stirring occasionally, until the tortellini are tender and cooked through.
Remove the pot from heat and stir in the grated Parmesan cheese and chopped fresh basil.
Adjust seasoning with additional salt and pepper if needed.
Serve immediately, garnished with extra Parmesan and basil if desired.