Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the sweet potato and slice into batons about as big as your little finger. Pop in a bowl, add 1 tbsp olive oil and some salt and pepper. Toss to coat in the oil and seasoning then spread out on a baking tray. Roast for 30 mins till tender and a little charred.
While the sweet potato roasts, slice half the radishes off the leaves and give them a good rinse. Finely slice them. Pop in a bowl. Finely grate in the lime zest and squeeze in the juice. Add a pinch of salt and scrunch everything together. Set aside to marinate.
Peel and finely slice the shallots. Warm a frying pan over a medium-low heat. Add 1 tbsp olive oil and the shallots. Gently fry for 10 mins, stirring often, till the shallots are golden. Pop them on a plate lined with kitchen paper. Set aside.
Drain the black beans and give them a good rinse. Peel and crush the garlic. Halve the chilli, scoop out the seeds and white pith for less heat (or leave them in for a spicier dish), and finely chop it.
Put the frying pan back on a medium heat. Add 1 tsp cumin seeds and fry for 30 seconds, then add the garlic and chilli. Fry, stirirng for another 30 seconds. Tip in the beans, add 2 tsp smoked paprika and 4 tbsp water. Gently heat for 2-3 mins, stirring, till the beans are warmed through.
Wrap the tortillas in foil parcel and pop them in the bottom of the oven for 3 mins to warm through. Finely shred the lettuce leaves.
Carefully unwrap the tortillas and arrange on 2 plates. Top with the lettuce, black beans and nestle a few roast sweet potato slices on top of each one. Scatter over the lime-marinated radishes, crumble over half the pack of feta and garnish with a few coriander leaves to serve.