Combine salt, pepper, and both cans of tomatoes together in a bowl with the onion and chili powder.
Sprinkle the roast with salt and pepper.
Heat olive oil in a large dutch oven and add the roast, browning 5 minutes a side.
Place roast in slow cooker and pour tomatoes over top. Cook on high for 6 hours.
Remove roast and slice it.
In a small bowl take a ½ cup of tomato juice and add and add one can of pinto beans. Mash well. Add back to slow cooker with remaining beans. Serve bean mash over pot roast.