Chickpea burgers in roasted portobello ‘buns’

Ingredients

• 2 x 250g packs portobello mushrooms
• 4 tbsp olive oil
• 1 x 400g tin chickpeas
• 2 tbsp tahini
• 1 tbsp gram flour (or plain flour)
• 1 x 31g pack coriander, chopped
• juice of ½ lemon
• ¼ tsp ground cumin
• ¼ tsp ground coriander
• ¼ tsp paprika
• 1 x pack Little Gem lettuce
• ½ avocado, sliced
• 1 small tomato, sliced
• a few slices of red onion, optional
• 1 x 150g tub coriander and mint raita, optional