To prepare the filling, combine 1 1/2 lbs (4 cups) of blueberries, 1/2 cup of granulated sugar, and 1 tablespoon of freshly squeezed lemon juice in a heavy medium saucepan. Use the back of a spoon or a potato masher to crush the berries gently. This step releases their natural juices, and the aroma is simply heavenly.
Stir the mixture over medium-high heat until the sugar is fully dissolved. Continue cooking until the mixture begins to boil and thicken, which should take about 7 minutes. You’ll notice the color deepening, and the filling will become syrupy.
In a medium bowl, whisk together 2 large egg yolks and 2 teaspoons of cornstarch. Gradually drizzle in half of the hot berry mixture while whisking constantly. This technique is crucial as it warms the yolks without cooking them, ensuring a smooth filling.
Return the mixture to the saucepan, whisking in the reserved hot berry mixture. Continue to stir over medium-high heat until it reaches a boil and thickens, about 3 minutes. The filling should be glossy and thick, looking utterly irresistible.
Next, whisk in 3 tablespoons of unsalted butter and 1 teaspoon of finely grated lemon zest. Stir until the butter is melted and everything is smoothly combined. Transfer this luscious filling to a heat-proof bowl, cover tightly with plastic wrap, and chill overnight. This resting period allows the flavors to develop beautifully.
The next day, it’s time to prepare the crust. Start by lightly buttering a 9-inch-diameter tart pan with a removable bottom to ensure easy removal later on. Combine 1 cup of all-purpose flour, 1/4 cup of sliced almonds, 2 tablespoons of granulated sugar, and 1/4 teaspoon of salt in the bowl of a food processor. Pulse until the almonds are finely ground.
Add 1/2 cup of chilled unsalted butter, cut into 1/2-inch cubes, to the mixture. Pulse several times until it resembles coarse meal, which should have a crumbly texture with small pieces of butter still visible.
In a small bowl, whisk together 1 large egg yolk and 1/4 teaspoon of almond extract. Add this mixture to the flour mixture and pulse until moist clumps form. Gather the dough into a ball, pressing it into the bottom and sides of the prepared tart pan. Be sure to pierce the crust all over with a fork; this prevents it from puffing up while baking. Freeze the crust for 30 minutes.
Preheat your oven to 375°F (190°C). Line the crust with a sheet of lightly buttered foil and fill with dried beans or pie weights. Bake for about 12 minutes, until the crust is set. Carefully remove the foil and weights, and bake again for another 16 to 18 minutes, or until the crust is lightly golden. It should be firm to the touch.
Once baked, place the crust on a wire rack to cool completely. When the crust is cooled, fill it with the chilled filling mixture, spreading it evenly. Arrange the remaining 4 cups of blueberries on top, creating a beautiful pattern. Sprinkle the remaining 1/4 cup of sliced almonds around the edges for added texture and flavor.
Cover the tart loosely with plastic wrap and chill for at least 2 hours, or up to 1 day before serving. This resting period will enhance the flavors and set the filling nicely.