Caramelized Onion, Potato and Cheddar Tarts

Steps

1. Make the pastry: whisk the flour with half of the salt. Set aside 20 g of the butter for the onions, then rub the remaining butter into the flour until the largest pieces are pea-sized.
2. Drizzle in the ice-cold water, tossing with a fork until the dough clumps. Divide in half, flatten into two disks, wrap and chill for 1 hour.
3. While the dough chills, melt the reserved butter in a large skillet over medium-low heat. Add the onion and a quarter of the remaining salt and cook, stirring often, 35–40 minutes until deep golden. Cool.
4. Simmer the potato slices in salted-free water for 4 minutes until just tender; drain and pat dry.
5. Preheat the oven to 400°F (200°C). Roll each disk into a 12-inch round, fit into two 9-inch tart or pie pans, prick the bases, line with foil and weights, and bake 15 minutes. Remove foil and weights and bake 5 minutes more.
6. Whisk the egg, milk, mustard, thyme, black pepper and the rest of the salt to make the custard.
7. Lower the oven to 375°F (190°C). Sprinkle half of the cheddar and half of the Parmesan over the two crusts, spread the onion on top, then shingle the potato slices over the onion.
8. Pour the custard evenly into both tarts, then scatter the remaining cheddar and remaining Parmesan over the tops.
9. Bake 30–35 minutes, until the topping is browned and the filling is set and reaches 160°F (71°C) in the center.
10. Cool on a rack for 20 minutes, then cut each tart into 6 slices.