Combine all the dry ingredients on a work surface or in a large bowl, then make a well in the center.

Add the cubed vegan butter, lemon zest, and eggs.

Mix with your fingertips until the mixture resembles coarse sand.

Knead lightly just until the dough comes together, then wrap it in plastic wrap, shape it into a ball, and chill for at least 1 hour.

To keep your hands clean, use a stand mixer fitted with the paddle attachment.

Gently warm the oat milk in a saucepan.

In a bowl, whisk together 1/3 of the warm oat milk (not hot), the egg yolks, sugar, vanilla, and cornstarch.

Whisk carefully until the cornstarch is completely dissolved.

Pour the pastry cream mixture into the remaining warm milk.

Cook over medium-low heat, whisking constantly, until thickened.

Remove the pastry cream from the heat.

Strain it to remove any lumps.

Add the citrus zests and the cold vegan butter.

Refrigerate until cool.

Roll out the chilled dough between two sheets of parchment paper to 3-4 mm thick.

Use rolling guides for an even thickness.

Using a rolling pin, transfer the dough to a tart pan or tart ring.

Press the dough firmly into the sides, then prick the base with a fork.

Trim the overhang.

Chill the dough in the refrigerator for 20 minutes.

Pour the chilled lemon pastry cream onto the prepared crust.

Roll out the remaining shortcrust pastry and lay it over the top.

Pinch the edges with your fingers to seal the tart.

Brush with egg wash, scatter the pine nuts evenly over the top, then prick a few holes in the crust with a fork.

Bake in a preheated 160°C oven for 55 minutes.

Increase the temperature to 180°C and bake for 8 to 10 minutes more.

Let the tart cool completely, then dust with icing sugar.

Slice and serve.