Combine pastry flour, sugar and salt. Rub or cut in cold butter until the mixture resembles coarse crumbs with small butter pieces. Add 3 tablespoons ice water and toss; add only enough extra water for the dough to hold together when pressed. Form a disk, wrap and refrigerate 30 minutes.

Roll on a lightly floured surface to about 12 inches across. Ease into the 9-inch tart pan without stretching, pressing gently into the corners. Trim the edge. Prick the base lightly with a fork and chill 30 minutes.

Heat the oven to 375°F. Set the pan on a baking sheet. Line the shell with parchment and fill with pie weights. Bake 20 minutes. Carefully remove weights and parchment; bake 7–10 minutes more, until the base looks dry and pale gold. Let cool about 10 minutes while making the filling. Reduce oven to 350°F.

Beat softened butter and sugar until lighter and creamy. Beat in eggs one at a time, then vanilla and optional almond extract. Fold in almond flour and the 15 g all-purpose flour until evenly combined. Do not taste uncooked filling.

Spread the jam thinly across the cooled shell. Spoon the almond filling over it in small dollops, then spread gently without dragging jam into the batter. Scatter sliced almonds over the top.

Bake 35–45 minutes, until the filling is set in the center, gently springy and golden; a skewer inserted into the almond layer should not carry wet batter. Check before removing even if the top has browned. Cover loosely with foil if the almonds or pastry edges brown too quickly.

Cool completely on a rack, approximately 1 hour, before removing the ring and slicing. Support the base while moving the tart. Cover and refrigerate leftovers.