Add butter and oil to Instant Pot. When it's hot, add the onions, carrots, and garlic. When there is some color on the onions (about 5 mins), add the roasted red peppers.
Next, add all the powder spices. Add a splash of beef stock to prevent burning. Sauté for another minute. Then add harissa paste and tomato paste.
Now turn off the Instant Pot and add the beef stock. Using a spatula, scrape the bottom of the pot to loosen up any bits stuck there. Then place the lamb shanks inside. Try your best to make sure the meaty parts of the lamb shanks are mostly submerged in the liquid.
Close the Instant Pot lid, turning the valve to "sealing" mode. Set to "pressure cook" on high for 45 minutes.
When done, either do a quick release or natural release. Carefully, using a big spoon or two, remove the lamb shanks (they will be soft) and transfer to a serving platter.
Using an immersion blender, blend the gravy into a smooth puree.
Add baby potatoes to the gravy. Turn the Instant Pot back on "saute" mode and cook uncovered, allowing for both the potatoes to cook through and for the gravy to reduce (about 15 minutes). Check to make sure the potatoes are done.
Pour the gravy and potatoes all over the lamb shanks. Garnish with parsley. Enjoy!