Place the lamb shanks on a cutting board or baking sheet. In a shallow bowl or plate, whisk together the salt, garlic powder, cumin, coriander, mustard and black pepper. Sprinkle the lamb shanks with the spice mixture and allow them to come to room temperature, about 30 minutes.
In a large Dutch oven, set over medium heat, add the vegetable oil. When hot, add the lamb shanks, in batches of two, searing them on both sides for about 5 to 7 minutes. Remove them from the pot and transfer them to a bowl. Repeat with the remaining two lamb shanks.
To a blender, add the chipotle in adobo, tomato sauce, water and salt. Blend until pureed, about 1 minute. Give it a taste and adjust salt to your liking, I added an additional pinch of salt.
To the Instant Pot, add the garlic, sliced onion, carrots and chipotle/tomato sauce mixture. Next, add the lamb shanks. Place the lid on the Instant Pot and manually set it for 45 minutes on “high pressure.” When the lamb shanks have finished cooking, allow the Instant Pot to naturally release for 15 minutes. Open the steam valve to release the remaining steam and once that’s complete, remove the lid and transfer the lamb shanks to a serving plate and cover with foil.
Turn the setting to sauté and allow the sauce to cook and thicken for about 10 to 15 minutes.
To serve, uncover the lamb shanks, pour the sauce and carrots over the shanks. Garnish with minced cilantro, sliced radishes, diced white onion and lime wedges.