Warm butter and oil in a multi-functional pressure cooker (such as Instant Pot®) set to Saute function. Add lamb and cook until browned on all sides, about 5 minutes. Transfer to a plate. Add onion and shallots to the pot; cook and stir until translucent, 4 to 5 minutes. Add garlic and cook for 1 minute longer.
Pour wine into the pot and scrape up any browned bits off the bottom of the pot. Cook until wine is reduced by half, 3 to 5 minutes. Place carrots and potatoes on top of onions. Season with rosemary and thyme; stir to combine well. Arrange lamb over vegetables; pour beef broth on top.
Close and lock the lid. Open vent for steaming. Select Meat/Stew function; set timer for 20 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer’s instructions, about 10 minutes. Meanwhile, mix water and potato starch until creamy.
Unlock and remove lid. Switch to Saute function. Stir potato starch mixture into the stew; cook until thickened, 2 to 3 minutes. Season with salt and pepper.
Use lamb leg instead of shoulder if preferred.
For an untraditional addition, you can dissolve 2 tablespoons of white miso in 1/4 cup of the beef broth for deeper umami flavor. Pour over the lamb with the remaining broth.
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