Caramelized Onion, Apple and Cheddar Tarts

Steps

1. Make the crust: whisk the whole-wheat flour, all-purpose flour and half of the salt in a bowl. Rub in the cold butter until pea-sized pieces remain.
2. Drizzle in the ice-cold water, tossing with a fork until the dough just holds together. Divide into two disks, wrap and chill for 1 hour.
3. While the dough chills, heat the olive oil in a large skillet over medium-low heat. Add the onion and the remaining half of the salt and cook, stirring often, for 35 to 40 minutes until deep golden and jammy.
4. Stir the thyme and cider vinegar into the onions, scraping the pan, then cool the onions for 10 minutes.
5. Preheat the oven to 400°F (200°C). Roll each dough disk to fit a 9-inch tart pan, press it in, prick the base with a fork and chill 15 minutes.
6. Line each crust with parchment and pie weights and blind-bake 15 minutes; remove the weights and bake 5 minutes more. Lower the oven to 375°F (190°C).
7. Brush the mustard over the warm crust bases, half in each.
8. In a bowl, whisk the egg, sour cream and black pepper. Stir in the cooled onions and half of the cheddar.
9. Spread half the onion mixture into each crust. Fan half of the apple slices over each tart, then scatter the remaining half of the cheddar evenly over both tops.
10. Bake 30 to 35 minutes, until the cheddar is bubbly and browned and the filling reaches 160°F (71°C) in the center.
11. Cool the tarts on a rack for 20 minutes, then cut each into 6 slices.