CRUST: Combine butter, sugars and a large pinch of sea salt in the bowl of a stand mixer fitted with paddle attachment. Cream until light and fluffy, about 3 minutes. Add egg and mix until well incorporated. Add vanilla and mx again. Add flour, not all at once, and mix on low spread just until dough comes together. Divide dough in half. Wrap each half in parchment paper and quickly flatten into 1-inch thick round discs. Refrigerate until firm, about 3 hours and up to 24 (dough can be wrapped in plastic and frozen after that for use later).
FILLING: Combine quince, rosehips (if using), lemon juice, ginger and 2 cups cold water in a saucepan. Set over high heat and bring water to a boil. Reduce heat, cover with a well-fitting lid, and let simmer until quince is quite soft, about 20 minutes - depending on size of dice etc. Add apples and continue cooking until the apples are soft, about another 20 minutes. If using rosehip jelly, now is the time to stir it in. Add a small pinch of salt. Keep an eye on things and make sure the fruit does not scorch. If liquid seems to have simmered off before fruit is completely soft add a splash or two more water as needed. Remove from the heat and let cool slightly. Pass the sauce through a folding mill and discard solids. Stir in maple syrup, adding more to taste if desired. Refrigerate to chill completely, about 1 hour.
Heat oven to 350. Remove dough from the refrigerator and let it stand for about 20 minutes to soften. Lightly dust a clean work surface and rolling pin with flour. Roll 1 half of dough into a 12 inch round, about 3/8th-inch thick. Brush off any excess flour. Transfer to parchment paper and set on a baking sheet. Quickly roll the 2nd half of the dough to approximately the same dimension. Spread chilled sauce evenly over the circle of dough already on the baking sheet, leaving about a 2-inch border all the way around. It is not necessary to use all the sauce - you want a generous filling but don’t go overboard or things will get messy. Whisk together egg yolk and heavy cream and brush the border with egg wash. Gently cover with the second circle of dough. Firmly press the outer edges of dough together, sealing in filling. Then fold up the edges and press to form a rustic pleated edge. Brush the entire thing with egg wash and sprinkle with a generous amount of sugar (2 tablespoons or something like that). Bake until golden brown, about 30 - 35 minutes. Reduce heat to 325 and bake another 10 minutes or so to make certain dough is cooked throughout. Let cool slightly before serving.
Recipe by Frances Boswell. Reprinted by permission.