I did this with black beans (instead of pinto) as a side dish for a dinner party featuring Mexican food. There's a fair amount of chopping, but aside from that it's mega easy -- love that it uses canned beans (which I drained but did not rinse, which I think is important for this one). I used a bit of olive oil to saute the onion mixture, and added some cumin, chili powder and Mexican oregano. I made it a few hours ahead of time up to the point of mixing in the beans (but not simmering), then right before serving I heated it through on the stove and moved it to a pre-warmed crock pot on the buffet. A single recipe makes A LOT more than 8-10 servings. We had 20 people at our dinner party and about 20% of the beans leftover at the end. And it's not because people didn't like them! There were lots of compliments and one person asked for the recipe. Non-meat eaters (like me) used the beans as their taco filling and it was delicious.
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