Preheat the oven to 180ºC/350ºF/gas 4.Trim, halve and wash the leeks, finely slice the green parts, then place in a large pan on a high heat with 1 tablespoon of oil and a pinch of sea salt and black pepper. Fry for 10 minutes, or until soft and sweet, stirring regularly.Peel and slice the garlic, add to the pan with the butter, and strip in the thyme leaves. Roughly chop the white part of the leeks and add to the pan, cover, reduce the heat and cook for 30 minutes, or until soft and sweet, stirring occasionally so the leeks don’t catch.Crack the eggs into a bowl, add the cream and a pinch of salt and pepper, and whisk until smooth.Lightly grease a 30cm x 40cm roasting tray with oil. Working fairly quickly, lay 4 sheets of pastry into the oiled dish, overlapping them in the middle and letting the edges overhang. Brush all over with oil, then coarsely grate some Cheddar around the overhang and inside. Finely chop the chives and sprinkle some in, then repeat, creating layers of filo with cheese and chives between them, and brushing with oil as you go (you’ll have enough for 2 or 3 layers, depending on how many sheets of pastry you’ve got).Check the leeks and season to perfection, then spoon into the filo-lined tray. Pour over the egg mixture and gently mix together, then scrunch and roll up the excess pastry to make a rim, and crumble over the feta.Brush the exposed pastry edges with oil, then bake on the bottom of the oven for 30 minutes, or until the pastry is golden and the filling is set.Thinly slice the apples and rub with lemon juice then, along with the watercress, dress with a little extra virgin olive oil and red wine vinegar, and season to perfection. Serve alongside the tart.