Pick over black beans, looking for any debris or mangled looking beans. Rinse well under cold water. In a large bowl or pot, add cold water until 3 inches above beans. Allow to soak overnight.
Rinse beans again. Add to a large pot and add cold water until 1 inch above beans. Peel garlic clove and add whole. Slice onion in half, remove skin and add ½ half to pot with beans. Finely dice the remaining onion and reserve.
Bring beans to a boil and then reduce to a simmer. Simmer for a least one hour, then test beans. They should be very soft but not falling apart. If beans feel firm or al dente, cook for another half an hour, or until they reach desired consistency above. Remove beans from heat and add salt, starting with one teaspoon. Let cool for about a half hour then taste beans. If you desire more salt, add in half-teaspoon increments until beans are to taste.
Using a slotted spoon, transfer beans along with the garlic and onion they were boiled with to a food processor or a blender, reserving the bean liquid. Blend until beans are very smooth, adding reserved liquid to thin if mixture is too thick (you will know if beans at the bottom are blended and the beans at the top of the blender are still whole).
In a large skillet, heat olive oil over medium-high heat. Add onions and sauté, stirring constantly, for 5 minutes or until crisp and golden brown. Add the blended bean. Turn down heat to medium and continue to stir occasionally while the liquid evaporates and the beans thicken. (About 15-20 minutes.) Add more oil if beans are sticking to the pan. When you can scrape a spoon against the bottom of the pan and the beans take 2-3 seconds to return, they’re ready. Remove from heat and transfer to a plate; serve with crema agria (sour cream).