Caramelized Onion, Mushroom and Cheddar Tarts

Steps

1. Make the pastry first: whisk the flour with 4 g of the salt in a large bowl. Add 225 g of the cold butter and rub or cut it in until the pieces are pea-sized with some flat flakes.
2. Drizzle in the ice water a little at a time, tossing with a fork, until the dough holds together when squeezed. Divide it in half, shape two flat disks, wrap, and chill for 1 hour.
3. While the dough chills, make the filling: melt the remaining 15 g of butter in a large skillet over medium-low heat. Add the onion and cook, stirring often, for 25 to 30 minutes until deep golden.
4. Raise the heat to medium, add the mushrooms and cook 8 minutes until browned and their liquid is gone. Stir in the garlic and thyme for 1 minute, then add the spinach and cook until wilted. Cool the filling.
5. Preheat the oven to 400°F (200°C). Roll each dough disk into a 12-inch round and fit into two 9-inch tart pans, trimming the edges. Prick the bases and chill 15 minutes.
6. Line the crusts with parchment and pie weights and blind-bake 15 minutes; remove the weights and bake 5 minutes more until dry and lightly golden. Lower the oven to 350°F (175°C).
7. Make the custard: whisk the egg, milk, mustard, black pepper, nutmeg and the remaining 3 g of salt in a bowl.
8. Divide the onion-mushroom filling between the crusts, sprinkle half the cheddar over the filling (half per tart split evenly), pour the custard over, then top with the remaining half of the cheddar.
9. Bake 30 to 35 minutes until the custard is just set in the center, reaches 160°F (71°C), and the cheddar is golden.
10. Cool the tarts on a rack for 30 minutes, then cut each into 6 slices.