Line a sieve with 3 pieces of kitchen paper, tip in the yoghurt, pull up the paper and very gently apply pressure so that the liquid starts to drip through into a bowl, then leave to drain. Halve the tomatoes, season with sea salt and black pepper, drizzle with 2 tablespoons of extra virgin olive oil and the vinegar, then toss and leave to macerate (this really brings out the flavour).
Preheat the oven to 190ºC/375ºF/gas 5. Peel and finely slice the garlic, then place in a large non-stick ovenproof frying pan on a medium heat with the fennel seeds and 2 tablespoons of olive oil. Fry for a few minutes, stirring regularly, while you prep the veg, adding to the pan as you go: trim, wash and slice the leek, scrub the potatoes, squash (deseed, if needed) and courgettes and chop into 2cm chunks. Cover and cook for 15 minutes, shaking the pan occasionally, then remove the lid, tip in the chickpeas (juices and all) and season lightly with a pinch of salt and pepper. Finely chop the preserved lemon, discarding any pips, then add to the pan with a drizzle of juice from the jar, and the harissa. Fry for a further 15 minutes, or until beautifully caramelised, stirring occasionally.
Meanwhile, cover the sour cherries and saffron with 300ml of boiling water, leave for a few minutes, then add to the pan with the tomatoes, reserving the macerating juices. Lay the filo out flat, then brush all over with the reserved tomato juices. Roughly scrunch, wave and layer the filo into the pan, partly tucking it in at the edges (there’s no need to be neat). Scatter over the sesame seeds and bake for 25 minutes, or until golden and crisp. Transfer the yoghurt to a plate and drizzle with a little oil from the harissa jar, then serve with the pie.