Preheat the oven to 180C/350F.
Scrub the sweet potatoes and place on a baking tray. Place in the oven and bake for 45 minutes until crisp on the outside and they can be pierced easily with a small knife.
Meanwhile, heat the 2 tablespoons of water in a large sauté pan over a medium heat.
When the water is bubbling, add the onion with a pinch of salt and cook until the onion is translucent. Add more water, a few tablespoons at a time, as needed.
Add the garlic, jalapeno, cumin, oregano/marjoram, red pepper flakes, salt and black pepper. Cook for 1 to 2 minutes, stirring frequently.
Add the can of chopped tomatoes. Simmer and cook with the lid off until most of the liquid has evaporated and the sauce is thick, about 5 minutes.
Add the cans of black beans with the liquid. Bring back to a simmer and continue cooking until the mixture is a thick sauce, about 10 minutes.
Stir in the lime juice and enjoy.
