Place the beef cubes and sliced onion in the slow cooker.
In a bowl, stir together the red curry paste, coconut milk, fish sauce, brown sugar, ginger, and garlic until smooth.
Pour the curry mixture over the beef and onion. Stir once to coat everything evenly.
Cover and cook on low for 8 hours, until the beef is fork-tender.
Stir in the lime juice right before serving.
Spoon the curry over bowls of jasmine rice and top with fresh basil leaves.