Juice one lime. Peel and grate or crush the garlic. Mix the juice and garlic with 1 tbsp olive oil, a pinch of chilli powder and a little salt. Add the pork. Turn it to coat in the marinade. Put to one side.
Quarter the tomatoes. Scoop out the seeds. Finely chop the tomatoes. Finely chop the chilli. Flick out the seeds for less heat. Finely grate or pare the zest from half the other lime. Juice one half. Finely chop half the mint leaves, saving the rest for later.
Peel the onions. Dice one onion. Mix the diced onion, tomatoes, chilli, lime zest and juice and chopped mint together with a pinch of salt. Taste and, if you think you'd like it spicier, add a pinch of chilli powder.
Separate the lettuce leaves from half the lettuce. Rinse them. Dry with a salad spinner or kitchen paper. Peel and finely slice the remaining onion.
Warm 1 tsp of oil in a large frying pan. Add one of the tortillas. Fry for 2 mins till it’s dark golden. Flip. Fry for another 1-2 mins. Put on a plate lined with kitchen paper. Fry the rest of the tortillas the same way, adding more oil when needed.
Warm another 1 tsp oil in the pan if it needs it. Add the sliced onion. Season. Fry over a medium heat for 5-8 mins till the onion is golden and a little crisp looking. Tip out onto a plate.
Add the pork to the pan. Fry for about 5 mins. Turn every so often till it’s golden and cooked through.
Top each crisp tortilla (tostada) with a handful of lettuce, some salsa, red onion, pork and the remaining mint leaves. Serve with the remaining lime sliced into wedges.