225g white beans
1.5L water
1/4 medium whiteo onion, roughly sliced
3 garlic cloves, roughly chopped
4 large sprigs cilantro
salt to taste
2 ancho chiles, veins and seeds removed, lightly toasted and soaked for 15 minutes
2 tablespoons lard or vegetable oil
2 tablespoons finely chopped white onion
85g queso fresco, crumbled, or Muenster, finely diced

To Serve:
4 serrano chiles, charred and roughly chopped
finely chopped cilantro

Pick over the beans, rinse well, and put into a large saucepan with the water, sliced onion, garlic and cilantro.
Cook over a low heat until tender, about 2.5 hours, depending on the age of the beans; they could be left to cook overnight in a slow cooker.
Add the salt and cook for about 10 minutes more.

Put 250ml of the bean liquid plus 125ml of the beans into a blender, add the chiles torn into pieces, and blend until smooth.
Heat the lard in a skillet, add the chopped onion, and cook without browning until translucent.
Add the chile puree and cook over a fairly high heat for about 2 minutes, scraping the bottom of the pan to prevent sticking.
Add this mixture to the beans and cook over medium heat until well seasoned, about 15 minutes.

Stir in the cheese, when melted, serve immediately.
Top with chiles and cilantro.