Heat oil in a large skillet over medium-high heat. Add beef, cooking in 2 batches, turning occasionally, until browned on all sides, 5-7 minutes.
Place browned beef in a lightly greased slow cooker. Add beef stock to skillet and stir with a wooden spoon to loosen browned bits from bottom of skillet. Transfer stock to the slow cooker.
Stir in curry paste, fish sauce, sugar, onion, cayenne pepper and curry powder. Cover and cook on low until beef is very tender, about 8 hours.
About 1 hour before serving, add coconut milk and cream to slow cooker and stir to combine.
About 10 minutes before serving, add green beans and cook until tender. Just before serving, turn off slow cooker and stir in cilantro and spinach. Stir gently until spinach begins to wilt. Stir in lime juice. Season with salt and pepper, to taste. Serve over rice or rice noodles, if desired.