For the berry filling: Add the sugar, pinch of salt and 2 (1/4-pints) blueberries to a saucepan. Zest the lemon over the top, then juice about 1 tablespoon and add to the berries. Give it a stir. Put on the stove about medium heat and cook until the berries release their juices and looks like jelly or jam.
While that's happening, whisk your egg yolks and cornstarch together in small bowl. Stir into the berries and boil until nice and thick. Set aside.
For the crust: Put your flour into a food processor, add the 1/2 cup almonds, the sugar, a pinch of salt and pulse until it's a fine meal. Add the egg yolk and cold butter (Cold butter makes the crust nice and flaky.) Add the extract and process the mixture until it turns into a little ball of dough.
Press the dough nice and evenly into a 9-inch tart pan with a removable bottom. Freeze 1 hour or so to firm it up. Freezing will also help make a nice crispy tart after baking.
Put a piece of parchment on the dough and fill it with dried beans. Bake for 12 minutes or so. This is called blind baking, which keeps the crust nice and flat and leaves plenty of room for the filling.
Remove the beans and parchment and cook the crust until it's nice and golden brown, another 16 minutes. Let cool.
Heat the black currant jelly and 1 tablespoon water in a saucepan to melt it.
Pour the blueberry filling in the tart and spread out to the edges. Mound the remaining fresh blueberries over top. Brush the top with the jelly glaze. Take the remaining toasted almonds and sprinkle them around the edges. Place in the fridge to firm up.