Place blueberries in heavy bottomed pot. Mix sugar and cornstarch together and sprinkle over blueberries, and heat over medium heat.
Add in balsamic and lemon juice, then bring the blueberries to a gentle boil, stirring often to ensure they cook evenly and don't stick.
Preheat the oven to 375*F. Roll out pastry and place in the bottom of the tart shell. Gently press in and trip the edges. Poke the pastry all over with a fork to prevent it from bubbly while baking. Place in preheated oven and bake 8-10 minutes, until just golden. Remove from oven and allow to cool
Remove blueberries from heat, when they are thick and bubbling. Set aside.
In a small heavy bottomed sauce pan, or in a double boiler, whisk together egg yolks, sugar and cream into a creamy mixture.
Stirring constantly over medium low heat, allow the custard to warm and start to thicken. When it coats the back of a spoon, remove from heat and allow to cool, about 5 minutes.Step 7 - To assemble, pour blueberry mixture into tart shell and spread evenly. Pour custard on top and allow to cool and set completely before slicing. Enjoy!
This tart is also amazing with raspberries, black currents, or blackberries too!