Heat a large griddle pan over a medium heat, or preheat the barbecue, if using. Mix 1 tablespoon of the oil with the crushed garlic and rub into the mushrooms and courgettes.
Griddle or barbecue the mushrooms for 5 minutes each side. Cook the courgettes for 3 minutes, turning halfway (if barbecuing, put them on a sheet of foil to stop them falling through the grills). If using a griddle pan, cook the veg in batches, keeping warm in foil as you go.
Meanwhile, make the salad: toss together most of the Italian salad with the tomatoes. Mix the harissa paste with the lemon zest and juice and 2 tbsp of water, then drizzle just a little over the salad.
Lightly brush the cut side of the baps with the remaining oil and griddle or barbecue for 1-2 minutes, until lightly toasted. Top the 4 bottom halves with a few of the reserved salad leaves, a mushroom, some griddled courgettes and then scatter over some feta. Drizzle with the remaining dressing, then top with the remaining bap halves and serve with the salad on the side.