Pat lamb dry and season on all sides with salt and pepper. Warm oil in a skillet over medium-high heat. Add lamb and cook, turning often, until browned on all sides, 5 to 7 minutes. (Work in batches if necessary. Do not overcrowd skillet or lamb will not brown properly.) Transfer to a slow cooker.
Add onions to skillet and cook, stirring often, until slightly softened, about 3 minutes. Sprinkle curry powder and flour over onions. Cook, stirring, 1 to 2 minutes. (Mixture will be very dry.) Pour in broth, increase heat to high and bring to a boil. Stir to scrape up any browned bits on bottom of skillet. Pour liquid in skillet over lamb in slow cooker. Cover and cook on high for 2 hours. Add carrots and raisins and cook 1 hour longer.
Use a slotted spoon to transfer lamb to a serving platter and surround with carrots and raisins. Taste sauce and add salt and pepper as needed. Spoon sauce over lamb and serve at once.