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, dried Italian seasoning, and crushed red pepper flakes; cook for 1 minute until fragrant.
Pour in the diced tomatoes with their juices and chicken broth; stir to combine.
Bring the mixture to a simmer, then add the cheese tortellini.
Cover and cook for 7-9 minutes, stirring occasionally, until the tortellini are tender and cooked through.
Reduce heat to low and stir in the heavy cream and grated Parmesan cheese until the sauce is creamy and well combined.
Season with salt and freshly ground black pepper to taste.
Remove from heat and sprinkle with chopped fresh parsley before serving.