This ancient traditional sauce has as many variations as the number of cooks who have made it over the past 8000-10000 years. Here's one to try:
Soak 2 ancho or other dried chilis by covering them with boiling water and letting them sit for 15 minutes.
Put in a blender:
1 small onion, coarsely chopped
2-3 garlic cloves
1 cup fresh pumpkin or squash seeds, cleaned of pulp
1 cup veggie or chicken broth
the soaked ancho chilis
1/2 tsp salt
juice of 1-2 limes (depending on size)
Whirl it all up in the blender till a thick uniform paste forms. To use it, add more broth and stew a chicken or game in it until cooked. Serve over rice or with steamed tortillas. Or use it to flavor soups by stirring the paste into the soup broth for flavoring. I haven't tried it with bean soup, but it sounds like a perfect combination.
There are infinite variations, so have fun with it. You can find dried chilies in most groceries.