Heat olive oil in a large deep skillet or Dutch oven over medium heat.
Add the Italian sausage and cook, breaking it up with a spoon, until browned and cooked through, about 5-6 minutes.
Add the diced 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 juice and chicken broth, stirring to combine.
Add the dried Italian seasoning and crushed red pepper flakes. Season with salt and black pepper to taste.
Bring the mixture to a boil, then reduce heat to medium-low.
Add the cheese tortellini to the pot and stir gently to submerge them in the liquid.
Cover and simmer for 7-9 minutes, stirring occasionally, until the tortellini are cooked through and the sauce has slightly thickened.
Remove from heat and stir in the grated Parmesan cheese.
Sprinkle chopped fresh parsley over the top before serving.