Step 1
Combine potatoes and water in a multi-functional pressure cooker (such as Instant Pot®). Layer sausages on top of the potatoes. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 12 minutes. Allow 10 to 15 minutes for pressure to build.
Step 2
Release pressure using the natural-release method according to manufacturer's instructions about 15 minutes. Release remaining pressure using the quick-release method about 5 minutes.
Step 3
Preheat the oven to 200 degrees F (95 degrees C).
Step 4
Unlock and remove the lid. Transfer sausages to a plate and potatoes into a large bowl. Break up the potatoes with a potato masher and stir in 1/4 cup butter until melted and incorporated Slowly mix in whipping cream until desired consistency is reached. Season with salt and pepper. Place mashed potatoes in the oven to keep warm.
Step 5
Wash out the pressure cooker pot. Return to the cooker and select Saute function. Melt 2 tablespoons butter. Cook sausages in the melted butter until browned on each side 2 to 3 minutes. Transfer sausages to an oven-safe container and keep warm in the oven.
Step 6
Deglaze Instant Pot® with 2 tablespoons of beef broth stirring and scraping the bottom of the pot to remove any brown bits. Add sliced onion and saute until onion is softened and browned 3 to 4 minutes.
Step 7
Whisk flour and 2 tablespoons broth together in a small bowl to create a slurry. Pour in remaining broth and whisk until well combined. Pour broth mixture into the pot and cook whisking frequently until gravy begins to thicken. Stir in browning sauce and season with salt and pepper.
Step 8
Place a scoop of mashed potatoes onto a plate top with two bangers and onion gravy and serve immediately.