Dry the lamb thoroughly with paper towels and score the fat in a cross pattern.
Cover the bottom of an insta-pot (pressure cooker) with olive oil, and add 1 tsp salt and a few grinds of black pepper. Place the lamb in fat-side-down and cover with the remaining ingredients. Sprinkle more olive oil, salt, and pepper on top.
Cook on high (meat setting) for 15 minutes. Allow pressure to release. Strain out the liquid and set aside. Cook for another hour on high. Add liquid to pan. Whisk in sprinkled xanthan gum, add butter, and bring up to a boil while whisking. Remove from heat. Salt and pepper to taste.
Remove lamb and allow to rest for 5 minutes. Cut lamb into bite-sized chunks.
Place a bit of sauce in a small bowl, add a few bites of lamb and some of the olive mixture. Serve immediately.