Mozzarella beef burgers with spicy fried onions

Name: Mozzarella beef burgers with spicy fried onions
Prep time: 30 min
Total time: 30 min
Cuisine: American
Ingredients: 2 x 250g packs rump steak (or 500g 15% fat Taste the Difference beef mince); 1 tsp Dijon mustard; 2 large black or regular garlic cloves, crushed; 3 tbsp plain flour; a pinch of hot chilli powder; a pinch of smoked paprika; 1 large onion, sliced thinly; 4 tbsp vegetable oil; 1 x 105g pack smoked pancetta (12 slices); 1 x 125g-150g ball mozzarella (drained weight); 6 tbsp mayonnaise; 2 tbsp ketchup; 1 tbsp sweet chilli sauce; a squeeze of lemon juice; ½ pickled cucumber, finely chopped (from a jar, we used Mrs Elswood); 4 seeded burger buns; a handful of shredded Cos lettuce; 2 large vine tomatoes, sliced; 3 pickled cucumbers, sliced (see above)