Heat 15 ml vegetable oil in a large frying pan over medium–high heat. Add the beef mince, breaking it up with a spoon, and cook for 5–7 minutes until browned. Drain off any excess fat.
Add the finely diced onion and cook for about 3–4 minutes, or until softened. Stir in the minced garlic, chilli powder, ground cumin, dried oregano, salt and black pepper, and cook for another minute until fragrant.
Pour in 60 ml water and simmer for 2–3 minutes, or until most of the liquid has evaporated. Check that the mince reaches an internal temperature of 71 °C.
Warm the tortillas to make them pliable: wrap them in a damp tea towel and microwave for 30–60 seconds, or briefly dip each tortilla into the hot frying oil for about 5 seconds per side. Drain on kitchen paper.
Place a warm tortilla flat on a board. Spoon about 2 tablespoons (30 ml) of the beef mixture onto one side, then roll it up tightly. Secure with a cocktail stick if needed. Repeat with the remaining tortillas.
Heat 720 ml vegetable oil in a deep pan or deep-fat fryer to around 180 °C. Fry 3–4 rolled tacos at a time, seam-side down, for 2–3 minutes on each side, or until golden and crisp. Remove with tongs and drain on a wire rack over kitchen paper.
Arrange the tacos on a serving platter. Top with shredded iceberg lettuce, crumbled feta cheese or grated Monterey Jack, soured cream or crème fraîche, and red or green salsa, plus diced avocado or guacamole if you like. Serve immediately while still warm and crisp.