Make the pastry. Add the flour, a little salt and pepper and the butter to a mixing bowl, then rub in the butter with your fingertips or using an electric mixer until you have fine crumbs. Add the basil, then mix in enough water to form a soft but not sticky dough.
Knead the pastry lightly, then roll it out thinly on a lightly floured surface. Stamp out 24 x 6 cm (2½ inch) circles with a plain biscuit cutter, then press into the buttered sections of 2 × 12-section mini muffin tins, rerolling the trimmings as needed. Chill for 15 minutes.
Fork the eggs and milk together in a bowl. Add the Parmesan, spring onions and a little salt and pepper and mix well. Spoon into the tarts, then add a slice of tomato to each one.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 18–20 minutes until golden and the filling is just set. Leave to stand for 10 minutes, then loosen the edges of the tarts and remove from the tins. Garnish with tiny basil leaves just before serving.