Preheat the Instant Pot on Saute HIGH. Move onto the next steps while the Instant Pot is preheating.
Mix the ground beef, 2 tbsp of French onion soup, egg, bread crumbs, salt and pepper together.
Use a cookie scoop to portion out the meat into equal balls. Roll the ball in the palm of your hands to create a very tight, uniform ball. Repeat with remaining meat.
The Instant Pot should read 'HOT'. If it doesn't, wait until the Instant Pot display reads 'HOT' before continuing.
Melt 2 tbsp of butter in the Instant Pot. Brown the meatballs in the Instant Pot. Brown on two sides, working in batches if needed.
While the meatballs are browning, wash and dice the potatoes into 1 inch cubes. Place in a steamer basket.
Once all of the meatballs have been browned, remove from Instant Pot. Deglaze the Instant Pot with the beef broth.
Add in the Worcestershire sauce and dried parsley. Place the meatballs in the liquid, pour the remaining French onion soup on to the meatballs.
Place the steamer basket on top of the meatballs.
Cook on Manual (or pressure cook) HIGH for 4 minutes. Quick release the pressure.
Remove the potatoes and mash with 1/4 cup of butter and milk. Season with salt and pepper to taste. Set aside.
Whisk together the water and cornstarch. Add to the Instant Pot, stirring until thickened. The gravy should be hot enough, but you may have to turn the Instant Pot onto saute for a moment to help thicken the gravy.
Serve the Salisbury steak Meatballs and gravy over the mashed potatoes.