Turn the Instant Pot to sauté and add oil.
Sauté the chopped onion until softened, about 3-4 minutes.
Add butter and let it melt. Stir in flour and cook for 1 minute.
Whisk in beef broth, scraping the bottom to deglaze.
Add heavy cream, Worcestershire sauce, and stir.
Place frozen meatballs into the pot. Stir gently to coat.
Seal the lid, set valve to sealing, and cook on high pressure for 5 minutes.
Allow natural release for 5 minutes, then quick release remaining pressure.
Stir and season with salt and pepper to taste.
Serve over mashed potatoes or egg noodles and garnish with parsley if desired.