Compote: Simmer berries (with the juice) with 1/3 cup sugar, lemon zest, lemon juice and salt in a medium pot over medium-high heat, until the berries burst and it looks saucy - about 10 minutes. Taste for sweetness and add extra sugar if necessary. Dissolve cornstarch in water to make a slurry. Stir slurry into bubbling compote and allow to thicken (it takes a couple of minutes) until it reaches the consistency of (lumpy) gravy and appears glossy. If not thick enough, mix up a little additional cornstarch slurry. If too thick, add a little reserved blueberry juice or water. Transfer to a bowl and chill.

Crust: The presentation is most elegant if you use a 12 inch fluted tart tin; removable bottoms, which are so easy to use. You can use a springform pan. In a stand mixer, or in a mixing bowl and using a hand mixer, cream together the butter, sugar and vanilla. Add flour and mix until combined. Remove from bowl and, on a floured sheet of plastic wrap, pat dough into a disc. With a well-floured rolling pin, roll dough into a circle an inch larger in diameter than your pan. The dough should be about 1/3" thick. Invert the dough into the pan, using the plastic wrap to help, and pat down to fit the bottom and come up the sides. Trim excess. Don't worry if the pastry breaks. Simply pat it back together. (Or divide the dough into 12 equal-sized pieces for individual tarts, or roll to fit into the 9" x 13" pan, taking the dough up the sides.) Chill crust in the freezer while you make the filling. No need to grease the pans; the pastry is buttery enough not to stick. Preheat oven to 350F.

Lemon filling: It's difficult to tell you exactly how many lemons you will need for this recipe. It depends on their size and juiciness. I generally use 6-8 large lemons, and find the zest from 3-4 of them is about 3 tbsp. Be sure to use fresh lemon juice. Zest the lemons first using a microplane or the finest setting on a grater. Then roll the lemons with your palm on the counter to yield more juice before you slice into them.
In a mixing bowl (with a pour spout if you have one), whisk together the sugar, salt, zest and juice. Next, whisk in the eggs and yolks until well incorporated. Whisk in flour until smooth (don't beat). Pour into chilled crust. Place baking pan(s) on a parchment-lined baking sheet and bake 45-55 minutes, until the crust is golden and the filling is set. (Individual tarts will bake in 25-35 minutes.) It's fine if there are a few brown flecks in the lemon filling. The point of a fine knife will come out mostly clean when done. Cool to room temperature and remove from pan (if using springform or fluted tart pan). Chill completely. Cut large tart into 12-16 wedges. If making bars, chill in the pan and cut. Serve topped with a generous spoonful of the cooled blueberry compote.