Heat a large, heavy-based saucepan or casserole dish over a medium heat and add the diced streaky bacon.
Fry the bacon for 8-10 minutes, until crispy. Use a slotted spoon to remove it and set aside, leaving 2 tablespoons of the bacon fat in the pan.
Add the chopped brown onion and green pepper to the pan and fry for 5-7 minutes, until the onion is translucent and the pepper has softened.
Add the minced garlic and cook for about 1 minute, until fragrant.
Stir in the chilli powder, ground cumin and smoked paprika, then cook, stirring, for about 30 seconds.
Add the drained pinto beans, undrained chopped tomatoes, chicken stock, soft brown sugar and apple cider vinegar. Stir well to combine.
Bring to a gentle simmer over a medium-high heat, then reduce the heat to low and simmer, uncovered, for 20-25 minutes, stirring occasionally, until the sauce has thickened.
Stir most of the reserved bacon back into the beans. Taste and season with salt and freshly ground black pepper as needed.
Serve the beans warm, garnished with the remaining bacon.