Drain beans, reserving liquid, and set aside. In a medium saucepan, cook
onion in oil over medium-high heat until onions are transparent, with slightly
browned edges, about 5 minutes.
Add chili powder, cumin, oregano, garlic and salt and pepper. Sauté for
1 minute, stirring constantly to prevent the spices from burning.
Add drained beans to the pot and cook on medium-low heat for about 1/2 hour.
If the beans seem too dry, add a little of the reserved bean juice, 1 or 2 tablespoons
at a time.
Remove from heat and mash mixture with a potato masher or large fork. Taste
for seasoning, and adjust if it needs more flavor.
For the sauce: Stir all ingredients together. If mixture seems too thick,
add a few tablespoons of water, until it is as thin as you like it. Heat mixture
slowly over low heat, as this sauce has a tendency to spatter. Cover and cook
until heated through, about 15 minutes.
To assemble: Spread each tortilla with a good spoonful of sour cream and
a few tablespoons grated white Cheddar cheese.
Meanwhile, heat a large skillet over low heat. Place the topped tortillas
on the skillet one at a time; cover and warm each about 2 minutes, or until
the cheese begins to melt. Or heat each tortilla about 30 seconds in a microwave
set on HIGH.
Roll a few spoonsful of beans inside each warmed tortilla; place on a plate;
top with rancheros sauce and more grated cheddar cheese.