Fill a 6-8 quart stock pot full of water.
Empty the beans out of the bag and put into the pan of water.
Let soak overnight.
Drain the water the next day.
Fill the pot with hot water.
Bring to a boil.
Once boiling, turn the heat down and simmer for the next six hours.
Add salt to the water after cooking an hour.
Test the beans to see if they are done.
Cook longer if needed.
Drain pan reserving 4 cups of water.
Chop onions and sauté in butter.
Grate the cheese and add to a bowl in a stand mixer. Add the sautéed onions as well.
Add drained beans to the bowl and beat in the mixer with a whip attachment.
Pour in bean water as needed to thin beans to a mashed potato consistency. Add salt if needed and serve with grated cheese on top.