Roll out the pastry on a lightly floured surface and line a 22cm (8½in) round x 2.5cm (1in) deep, loose-bottomed fluted flan tin, gently pressing the pastry into the flutes on the sides. Trim the pastry edges (see Cooking tips). Prick the base lightly with a fork, then chill for 10 minutes. Preheat the oven to gas 6, 200℃, fan 180℃.
Line the pastry case with foil, fill with baking beans, put on a baking sheet and bake for 20 minutes until the pastry has set. Remove the foil and beans and bake for another 5 minutes, until the pastry is turning pale golden.
Meanwhile, prepare the filling. Heat the oil in a frying pan. Add the spring onions and stir fry for 3-4 minutes until softened and tinged browned but keeping their fresh colour. Blend the mustard and crème fraîche together, then gradually mix in the eggs. Stir in the milk, then season with a little salt and a grinding of pepper. Tip all but a handful of the cheese and all the spring onions into the cooked pastry case. Scatter over 1½tsp of the tarragon. Pour in the egg mixture then scatter another 1½tsp of tarragon and the rest of the cheese on top.
Put the tart in the oven then lower the heat to gas 5, 190℃, fan 170℃. Bake for 25-30 minutes, until softly set. Remove and let the tart sit for about 15 minutes. While the tart is resting, lay the halved tomatoes on a baking tray cut-side up. Drizzle with a little oil, scatter over the rest of the tarragon and a grinding of pepper. Roast for about 8-10 minutes, or until just starting to burst. Serve on salad leaves with the tart.
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