First, make the spice mix. Toast the cumin seeds, coriander seeds and peppercorns in a small, dry frying pan, then tip into a spice grinder or use a pestle and mortar to crush to a rough powder. Add the remaining spice mix ingredients, except the cornflour, then grind again. Stir through the cornflour and a pinch of salt. The spice mix will keep in an airtight jar for up to one month.
For the pickled onions, toss the onions with the lime juice and a pinch of salt in a medium glass bowl, then set aside to soften.
Heat the oil in a frying pan over a medium-low heat, and crumble in the mince. Cook for 15-20 mins. At first, it will release some liquid, but once that has bubbled away, the mince will sizzle in its own fat and start to brown and crisp. At this point, stir or toss the mince with a wooden spoon so it browns and caramelises all over. When the mince is well toasted, sprinkle over half the spice mix and continue to cook for 2 mins. Pour over the beef stock and reduce the heat to a simmer. Cook gently for 10 mins until the beef has absorbed most of the stock, and is in a rich, thick gravy. Taste for seasoning.
To assemble the tacos, toast the tortillas in a hot, dry frying pan, or over a flame until lightly blistered. Transfer to a serving plate and spoon the mince into a bowl, then serve the other ingredients in separate bowls, so everyone can assemble their own. Alternatively, spoon the mince into the tortillas, then top with the avocado, tomato, cheese (if using), pickled onions and crushed tortilla chips, if using. Serve the lime wedges on the side for squeezing over.
