To get ahead, prep the cauliflower and chickpea tray so it is ready to go into the oven. The lamb can be cooked in advance and heated up to serve. The onion and parsley can be prepped (don't add the lemon juice) and the sauce made well in advance. Then all you need to do is cook the cauliflower, reheat the lamb and plate up.
This is completely delicious with large amounts of flatbreads but as per usual with my hungry teenage boys I have to double the recipe!
Adapted from a recipe by Georgina Hayden