Cook Italian sausage with diced onion. At the end of cooking, add minced garlic; cook until fragrant.
In a separate bowl, mix together chicken broth, heavy cream, cornstarch, sun-dried tomatoes, red pepper flakes, garlic powder, and Italian seasoning. Whisk well.
Slow Cooker
Add ravioli to the slow cooker. Add sausage mixture. Pour the liquid mixture over the ravioli and sausage. Top with butter.
Cook on low 3–4 hours or high around 2 hours.
Mix after an hour.
Once cooked through, add Parmesan cheese and stir until cheese is melted.
Stovetop
Cook ravioli in a large pot according to package instructions. Drain.
Reduce heat and return ravioli to the pot. Add sausage mixture and pour liquid mixture over the top.
Cover and simmer for 10 minutes, stirring halfway through.
Add Parmesan cheese and stir until cheese is melted.