Heat olive oil in a large deep skillet or Dutch oven over medium heat.
Add Italian sausage and cook, breaking it up with a spoon, until browned and cooked through, about 5-7 minutes.
Add chopped onion and cook until softened, about 3 minutes.
Stir in minced garlic and cook for 1 minute until fragrant.
Pour in chicken broth and diced tomatoes with their juice, stirring to combine.
Add heavy cream, dried Italian seasoning, and red pepper flakes. Stir well.
Bring the mixture to a gentle boil, then add the cheese tortellini.
Reduce heat to medium-low and simmer uncovered, stirring occasionally, until the tortellini are tender and the sauce has thickened, about 10 minutes.
Season with salt and black pepper to taste.
Remove from heat and stir in grated Parmesan cheese until melted and the sauce is creamy.
Garnish with chopped fresh parsley before serving.