Halloumi aubergine burgers with harissa relish

INGREDIENTS

• 2½ tbsp olive oil
• 2 onions , very finely sliced
• ½ aubergine , cut into 8 round slices
• 250g block halloumi cheese , cut into 8 slices
• 1 tbsp soft brown sugar
• 1 roasted red pepper from a jar, chopped
• 2 tsp harissa
• 4 ciabatta rolls, halved and lightly toasted
• 4 tbsp houmous