Heat oven to 220C/200C fan/gas 7. Put the sliced onion, lime juice and seasoning in a bowl, and set aside.
Heat 1 tbsp of the oil in a pan and fry the chopped onion and garlic until tender. Stir in the cumin and fry for 1 min more. Add the tomato purée, chipotle paste and beans, stir, then tip in half a can of water. Simmer for 5 mins, season, then roughly mash to a purée. (You can cook for a few mins more if it is a bit runny, or add a few splashes of water to thin.)
Meanwhile, brush the tortillas with the remaining oil and place on a baking sheet. Bake for 8 mins until crisp. Spread the tortillas with the bean mixture. Mix the radishes and coriander with the pickled onions, then spoon on top. Serve with lime wedges.