Place the beef cubes, diced onion, and minced garlic in the slow cooker.
In a bowl, stir together the coconut milk, red curry paste, fish sauce, and brown sugar until smooth.
Pour the coconut milk mixture over the beef and vegetables in the slow cooker.
Cover and cook on low for 8 hours.
Stir in the sliced bell peppers during the last 30 minutes of cooking.
Turn off the heat and stir in the fresh basil leaves just before serving.