Pre-heat the oven to 180C. On a floured work surface, roll the pastry out to half a centimetre thick. Use pastry to line the tart tin, pushing it into the corners with a ball of spare pastry. Trim the edges but let them overhang a little (you can trim them again after baking; this stops the case shrinking). Line with greaseproof paper and fill with baking beans, then bake for 15 minutes. Remove the beans and paper and bake for another 5 minutes, until the case is golden brown. (Tip: it helps to put the tart tin on a removable oven tray while baking, so you can just remove the whole tray from the oven when filling the tart, rather than having to precariously balance a full tart tin).
Meanwhile, prepare the filling. Heat the oil in a large sauté pan and gently fry the onion until softened. Slice the spinach leaves into 1cm strips, then add to the pan and cook over a medium-high heat until it has lost all of its water and is dark green and wilted (around 10 minutes). Season the spinach well.
In a jug, beat together the eggs, milk, nutmeg, the leaves of half the lemon thyme, and a generous amount of salt and pepper. Remove the baked tart case from the oven (see tip above) and trim off any overhanging pastry with a sharp serrated knife. Spoon the spinach mixture into the tart case, spreading it out evenly, then pour over the milk and egg mixture.
Crumble over the goat’s cheese, sprinkle over the tomatoes and the rest of the lemon thyme leaves, then scatter over the pine nuts.
Bake for around 30 minutes, or until the filling has set. Remove from the oven and leave to cool before serving, sprinkled with a little more freshly grated nutmeg and lemon thyme, if you like.