Heat your oven to 200°C/Fan 180°C/Gas 6. Peel and thinly slice the onion. Put it in a large bowl. Finely grate the zest from the lemon and set aside. Squeeze the lemon juice into the onion bowl. Add a pinch of salt and scrunch. Set aside to pickle.
Take 2 tortilla wraps and place them in 2 small ovenproof bowls (see step 3 if you don’t have ovenproof bowls). Place in the oven and cook for 5-10 mins. They should be golden and hard, taking the form of the bowl. Leave to cool in the bowls.
While the taco bowls bake, cut the remaining tortilla wraps into triangles and lay in a single layer on a baking tray. Bake on the shelf below the taco bowls for 8-10 mins till golden and hard. If you don’t have ovenproof bowls, you can skip the above step and cut all the wraps into tortilla chips.
Meanwhile, finely chop the tomatoes and add to the onions. Finely chop the chilli, flicking out the seeds for less heat. Add to the bowl, too. Set aside.
Peel and coarsely grate the carrot into a separate bowl. Finely chop the coriander stalks and add to the carrots. Zest the lime and set aside. Squeeze in half the lime juice. Toss together.
Halve the avocado and scoop out the flesh. Pop into a bowl. Add all the lime zest and the remaining juice to the avocado. Mash with the back of a fork till smooth. Season.
Peel the mango and cut the flesh away from the stone. Cut the mango flesh into thin slices. Set aside. Finely shred the romaine lettuce. Drain and rinse the black beans.
Heat a small pan for 1 min. When warm, pour in 1 tbsp oil. Tip in the black beans and heat gently for 5 mins. Pour in the pickled onions, the tomatoes and chilli, along with any juices from the bowl. Stir together on the heat for 2 mins to warm through.
Place a taco bowl on each plate. Divide the lettuce between the bowls. Top with the beans and carrot. Top with the mango slices and a dollop of zingy mashed avocado. Scatter over the coriander leaves and lemon zest. Serve with the baked tortilla chips for scooping.