Saute onion and garlic in the canola oil. Add the curry paste and stir until it's fragrant and covers the onion and garlic. Pour in the coconut milk and deglaze the pan. Stir in the fish sauce, lime juice, and sugar. Bring to a boil.
Transfer the sauce and beef to the slow cooker. Cover and cook on low for at least six hours. The beef should be tender, and the sauce should be thick but still fluid.