Combine maple syrup, black beans and ginger in a bowl and rub all over ribs. Cover tightly with plastic wrap and refrigerate for 4-12 hours, tossing once or twice to make sure they marinate evenly.
Preheat oven to 200°C/400°F/Gas Mark 5. Place ribs and marinade into a roasting dish just large enough to hold them. Add onions, garlic, soy and water and cover tightly. Bake 50 minutes, then remove cover and turn the ribs over. Cook a further 45-60 minutes, basting with cooking juices, until ribs are dark brown and caramelised and the meat is beginning to come off the bones.