Grease your tart tin with butter, and line with your chilled rolled shortcrust pastry.
Spread your jam into the bottom of the pastry crust.
Mix your frangipane ingredients: almonds, flour, mahlab, sugar, butter, egg and egg white, and lemon zest, thoroughly - either by hand or in a stand mixer. Pour over the jam and place in the refrigerator for one hour.
Heat your oven to 350°F (175°C) and bake the tart for 45-55 minutes with the tart tin placed on top of a baking sheet, until the top is golden and a toothpick pulled from the center is clean. Cool on a wire rack for two hours, then ice if desired.