Whisk the flour, sugar, salt, and lemon zest in a mixing bowl. Rub in the cold butter with your fingertips for 3 to 4 minutes, until the mixture looks like damp sand.

Add the whole egg and 1 yolk and press until the dough just holds together. Don't knead it, since overworking makes the crust tough. Shape into a disk, cut off one-third, wrap both pieces, and chill for 60 minutes.

Heat the oven to 180 C / 350 F with a rack in the lower third. Roll the larger piece between two sheets of parchment to 4 mm thick. Lay it in a 24 cm / 9.5-inch fluted tart pan with a removable bottom, press into the corners, trim the overhang flush, and prick the base with a fork.

Stir the lemon juice into the preserves. If the jam pours off the spoon in a ribbon, simmer it in a 2-quart / 1.9 L saucepan for 5 minutes and let it cool while you roll the lattice. Spread the jam in an even layer about 8 mm deep.

Roll the smaller piece with the trimmings to 4 mm and cut 8 to 10 strips about 1.5 cm wide. Lay half across the jam, then lay the rest at an angle to form a diamond lattice. Press the ends into the rim, trim, and patch any tears. Whisk the remaining yolk with the milk and brush it over the lattice and rim.

Bake for 40 minutes, until the pastry is deep golden and the jam bubbles slowly in the gaps. Cool in the pan on a wire rack for 60 minutes, then unmold, dust with powdered sugar if you like, and slice.