Drain and rinse soaked beans and put into large pot, cover with cold water. Bring beans to a boil, reduce heat and simmer until soft (1-2 hours).

In a large heavy pot (use one that has a lid) heat oil and add onions, garlic and mustard powder; sweat until onions are translucent. Add cider vinegar, stirring to release any bits from bottom of pan, add tomato puree, sugar, cloves and tamarind. Simmer, allowing flavors to combine, for approximately 20 minutes.

Use an immersion blender to blend all ingredients into a smooth sauce (you can also use a blender, but be careful, this is a hot liquid!). If a very smooth consistency is preferred, strain sauce through fine mesh or a fine sieve and return sauce to your heavy pot (add water if the sauce seems too thick). Add maple syrup, stir well, then add cooked beans. Cover the pot and place in 300 F oven for 1-2 hours, stirring occasionally to check consistency of sauce.