Cut the cauliflower into 4 steaks. Cut the onion into wedges, then the red pepper into large chunks.
In a bowl, mix the oil, paprika, ground cumin, lemon juice, salt and pepper.
Brush the cauliflower steaks, pepper and onion pieces with this mixture then put them on a hot griddle pan or barbecue and grill until tender and lightly charred.
Meanwhile, cut the buns in half and lightly toast or grill them.
Spread some pesto on the bottom halves of the buns, then add some salad leaves, the cauliflower steak, pepper and onion pieces.
Drizzle with some chilli sauce or mayo, scatter some fresh coriander leaves over it, then top with the other half of the bun and enjoy.