Heat your oven to 180°C/Fan 160°C/Gas 4. Light your barbecue if you’re using it. Pull the leaves and silks off the sweetcorn cobs. Simmer the cobs in a large pan of boiling water for 5 mins. Drain and leave the cobs to cool slightly.
Cut 2 of the tortillas into triangles (you can freeze your leftover tortillas for later). Lay them out in a single layer on a baking tray. Bake in the oven for 10 mins till they’re crisp. Set aside. Heat your grill if you’re not using a barbecue.
Put a dry frying pan over a medium heat. Tip in 1 tbsp of the pumpkins seeds, ½ tsp salt and a small pinch of the cayenne. Dry fry for 2-3 mins till the seeds pop. Shake the pan while they cook. Set aside.
Halve the avocado. Discard the stone. Scoop out the flesh and roughly chop it. Roughly chop the tomatoes. Drain and rinse the black beans. Mix them all together in a bowl. Zest and juice the lime. Reserve the zest. Pour in the lime juice with ½ tbsp oil. Gently mix together.
Pick the leaves from the coriander and finely chop the stalks. Spoon the yogurt into a bowl. Stir in the chopped coriander stalks and lime zest. Season. Tear the lettuce leaves.
Mix ½ tbsp oil with a pinch of the cayenne. Lay the sweetcorn cobs in a large dish and pour over the spicy oil. Rub all over the cobs to coat. Pop them on the barbecue or under the grill for 10 mins. Turn regularly till charred.
Spoon the yogurt over the cobs and sprinkle with the toasted pumpkin seeds. Serve with the black bean guacamole, toasted nachos and lettuce leaves. Scatter coriander leaves over everything to garnish.