Basting sauce: Mix all the ingredients together.Start by cutting off the top and bottom of each zucchini. Then slice the zucchinis crosswise into 3mm to 5mm rounds.
Brush both sides of the zucchini slices with a little olive oil and season to taste with salt and freshly ground black pepper.
Without overlapping them, place the zucchini rounds into a large frying pan (or two large frying pans to speed up the process) and grill over a medium-high heat for about 6 minutes or until slightly crispy but still tender.
Get started on the beef whilst the zucchinis are cooking. In a small bowl, combine all the Mexican rub spices together.
Slice the rump into small cubes roughly 1cm x 1cm.
Generously apply the Mexican rub to the meat messaging all the spices into the beef cubes.
Heat a splash of olive oil in a large frying pan over a high heat and cook the rump for 2 to 3 minutes or just long enough to caramelise the outside of the cubes and keep the centres juicy.
To serve, top the zucchini chips with the beef bits and sprinkle the grated cheese, red onion and fresh chilli over the top. Finish these nachos off with a squeeze of fresh lime juice and serve immediately.