Wash the potatoes and prick them with a fork all over, put in the microwave for 10 minutes until mostly cooked through. Preheat the oven to 200C/400F.
While the potatoes are in the microwave, make the chimichurri sauce by combining all ingredients with a pinch of salt in a food processor or a blender and pulsing until they turn into a sauce. Add more salt if needed.
Now take the potatoes out of the microwave and brush with olive oil, then put in the oven directly on the rack and cook for 15-20 minutes.
Meanwhile, make the black bean filling. In a large frying pan heat the olive oil and add chopped onions and red bell pepper, cook over low heat for 10 minutes until the onions are soft and started to caramelize. Add the minced garlic and cook for half a minute longer, then bring in the drained black beans to the pan together with cumin, oregano, smoked paprika and red wine vinegar with a pinch of salt. Stir everything to combine, then add the water and tomato paste, bring to a boil, then lower and heat and let the mixture simmer for 7-10 minutes, then take a potato mashed to the pan and mash about a quarter of the mixture, stir to combine and take off the heat.
Cut slits into each potato but do not cut all the way through, top with the black bean mixture and drizzle with some chimichurri sauce.