First, make the blueberry sauce. Mix the cornstarch and lemon juice in a bowl or cup and set aside. Heat the blueberries and date syrup in a small saucepan over medium heat for 3 minutes, stirring occasionally. Mash them as you go with a fork. Add the cornstarch mixture and cook for an additional 2 minutes. Continue to stir and mash the mixture as it thickens, then remove from the heat and set aside to cool.
- Preheat the oven to 350F.
Next, make the crust. Mix the melted butter, date syrup, vanilla, and salt together in a medium bowl. Add the flour and stir until combined. The dough should be greasy and very thick. If it feels too crumbly, add a few tsp of water at a time until it forms the proper dough. If it feels too sticky, use a few tsp of flour until it’s workable. Using your hands or a spatula, press the dough firmly into a 9-inch tart pan, making sure the layer of crust is even around the bottom and sides.
- Bake for 15 minutes or until the edges are just slightly browned. Remove from the oven, and use a fork to poke holes all along the top of the crust, being sure not to poke all of the way through. Set aside.
- Now, make the filling. Whisk the ingredients together until combined, and pour into the tart pan. Stir the blueberry sauce gently, and then drop spoonfuls of the blueberry sauce into the lemon filling until you’ve used about ¼ of the sauce. Reserve the rest, and take a knife or toothpick and swirl the sauce and filling together until swirled to your satisfaction. You can gently tap or shake the tart pan to make sure the filling settles into the crust.
Bake for 40-45 minutes, until the center of the tart jiggles slightly when you tap the pan but it shouldn’t slosh or look like liquid, just sticky on top. Bake longer if needed but if you overbake it, the filling will be rubbery, whereas if you underbake it a little it should be fine after setting up for a while.
- Remove the custard and place it, still in the pan, on a cooling rack. Cool to room temperature before placing it in the fridge, uncovered, for 2 hours or cover it and let it sit up to 1 day.
Once it’s chilled, you can remove the sides from the pan. Top with a layer of the remaining blueberry sauce, slice, and serve.
