Start the Instant Pot using the sauté feature set on high for 20 minutes (you will not use this feature for 20 minutes but it gives you freedom to cook at your own pace.
Add the olive oil to the Instant Pot insert and heat for 2 to 3 minutes. Add the lamb and brown for 5 to 8 minutes, stirring occasionally as to not burn the bottom of the pan.
Remove the lamb and set aside. Add the onions, carrots, tomato paste, salt and pepper to the Instant Pot. Sauté for 2 to 3 minutes. Add the beef stock, red wine and bouillon cube. Deglaze the pot by scraping up any pieces stuck to the bottom. Return the lamb with its juices to the Instant Pot, stir to combine.
Place the cornstarch into a small bowl and top with a few ladles of hot cooking liquid. Stir until the cornstarch is dissolved. Add this into the Instant Pot and stir.
Before sealing the lid, make sure that the bouillon cube has dissolved. Seal the Instant Pot and cook at high pressure for 20 minutes. Allow the pressure to naturally release for 10 minutes, then manually release any remaining pressure. Stir in the vinegar.
Serve the stew in warm bowls alongside mashed potatoes.