Heat a large skillet over medium-high. Add oil; swirl to coat. Add beef, and cook in 2 batches, turning occasionally, until browned on all sides, about 6 minutes. Place browned beef in a 5- to 6-quart slow cooker. Add stock to skillet, stirring and scraping to loosen browned bits from bottom of skillet; transfer mixture to slow cooker. Add curry paste, fish sauce, sugar, and onion; stir to loosely combine. Cover, and cook on low until beef is very tender, about 8 hours. Add coconut milk and haricots verts to slow cooker. Increase heat to high; cook until haricots verts are tender, about 12 minutes. Turn off heat; add cilantro leaves, spinach, and lime juice. Stir gently until spinach starts to wilt. Garnish with cilantro sprigs, if desired.