1. Preheat your grill to high. Peel and finely chop the garlic cloves. Halve the chilli, scraping out the seeds and white pith if you prefer less heat, then finely chop it. Finely chop the coriander stalks (keep the leaves for later). Finely chop the tomatoes.
2. Pour 1 tbsp oil into a pan and warm over a medium heat. Slide in the chopped garlic, chilli and coriander stalks. Stir in the fajita seasoning and fry for 1 min. Slide in the chopped tomatoes and cook for a further 2–3 mins till the tomatoes are just starting to soften.
3. Tip the black beans and the liquid from their tin into the pan and stir well. Season with a pinch of salt and pepper. Cover the pan with a lid, lower the heat a little and simmer for 8–10 mins, stirring occasionally, till thickened.
4. Meanwhile, peel and thinly slice the red onions. Slide into a small bowl. Grate in the lime zest and squeeze in the juice. Season with a pinch of salt and pepper, and scrunch everything together with your hands. Set aside for 10 mins to pickle lightly.
5. Coarsely grate the cheddar cheese. Halve the avocado, scoop out the stone, then finely chop the flesh.
6. Arrange the tortilla chips in a heatproof dish. Taste the black beans and add a pinch more salt and pepper, or a pinch of sugar, if you think they need it. Spoon the black beans over the tortilla chips and sprinkle over the cheese. Slide under the grill for 2–3 mins till the cheese has melted.
7. Remove the nachos from the heat and top with the pickled red onion, the diced avocado and the coriander leaves. Serve straight away.
8. Get Ahead
You can make the black bean sauce the night before you want to serve it. Follow the recipe up until the end of step 3, then transfer the black beans to a tub and store in the fridge. To make the nachos, reheat the black beans in the microwave or on the hob, then spoon them over the tortilla chips and continue the recipe.