Beef tacos

Name: Beef tacos
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Cuisine: Mexican
Ingredients: 1 tbsp sunflower oil; 500g beef mince; 300ml beef stock; 2 tbsp cumin seeds; 1 tbsp coriander seeds; 1 tsp black peppercorns; 1 tsp chipotle chilli flakes; 1 tbsp paprika; 1 tbsp dried oregano; 1 tsp garlic granules; 1 tsp onion granules; 2 tsp cornflour; 1 large red onion finely sliced; 1 lime juiced, plus wedges to serve; 6-8 small soft wheat or corn tortillas; 1 avocado stoned, peeled and chopped into small chunks; 1 tomato chopped into small chunks; handful of feta crumbled, or use grated mild cheddar (optional); 2 handfuls of tortilla chips crushed (optional)