Tuna Ceviche with Ginger, Chilli and Corn

What you will need

500 gm tuna, cut into 5mm dice

1 tsp very finely chopped ginger

½ Spanish onion, finely chopped

Juice of 4 limes

3 tsp extra-virgin olive oil

2 wild green chillies (or green birdseye chillies), finely chopped

1 bunch coriander, leaves only

1 small orange sweet potato (about 300gm), peeled, cut into 5mm dice, boiled until tender

Kernels from 1 corn cob

12 crisp tostadas

1 spring onion, sliced crossways