How to Make It
In the bowl of a food processor, combine the flour, sugar and salt. Pulse to mix. Add the butter and egg yolk and pulse until the butter forms peas sized pieces with the flour.
Place the crumbly dough into one 4-by-13-inch tart shell and gently press the dough against the bottom and walls of the pan. Make sure to evenly coat the sides while being careful to not compact the dough too much. Freeze the tart shell for 20 minutes.
Preheat the oven to 375°. Prick the bottom of the tart crust a few times with a fork, line the bottom with a sheet of tin foil, and place pie weights onto the lined foil. Bake until the edges are lightly golden, about 20 minutes. Let the pastry crust cool on a wire rack.
To prepare the filling, add the almonds to the bowl of a food processor and pulse until finely ground. Add the butter and sugar to the bowl and process until smooth. Then add the egg, reserved egg white, lemon zest, flour, cornstarch, salt and extracts. Pulse into an even consistency and fold in the chia seeds.
Lower the oven temperature to 350°. Remove the baking weights and tin foil from the tart pan. Spread the frangipane into the cooled shell. It should come up to just 1/4-inch in an even layer just below the top edge. Slice the peaches 1/8-inch thick and place them in neat bundles in a decorative pattern across the filling. Gently press the fruit into place. Place the tart on a baking sheet and bake until the crust and the frangipane filling are golden and have set, about 40 minutes. Cool on a wire rack. Slice and serve with a sprinkling of a few chia seeds.