Heat the olive oil in a large skillet over medium-high heat.
Season the lamb with salt and pepper. Add olive oil to a skillet and brown the lamb on all sides, about 3-4 minutes per side.
Layer the onion on the bottom of a 6-quart slow cooker, then place the lamb on top. Sprinkle the garlic and rosemary on top of the roast and pour in the beef stock. Cook on low for 6-8 hours, until the lamb is extremely tender.
Remove the lamb from the slow cooker. Place on a serving plate, and keep warm. Strain the liquid into a saucepan and bring to a simmer. In a cup, dissolve the arrowroot powder in 2 tablespoons of warm water, then add to the saucepan. Simmer, stirring occasionally, until the gravy has thickened, about 5-6 minutes.