Put two cups of blueberries in a small pot with the sugar.

Mix the water and cornstarch in a small cup and add to the pot with the blueberries.

Over medium heat, stir and cook until the sauce thickens and the berries begin to burst and extrude their juice, about 5 minutes. Remove from heat and gently mix in the remaining 1 cup of berries. Allow to cool.

Grate the lemon zest first; use the skin of a fresh, clean lemon, using only the yellow part, avoiding the white pith. Then squeeze the lemons for their juice.

Strain the yolks through a fine seive into a small pot and mix with the sugar.  Add the lemon juice. Cook on medium heat until the mixture begins to thicken. Do not boil. Turn off the heat and whisk in the butter pieces. Whisk in the lemon zest. Allow to cool in the refrigerator.

Bake a tart shell or use a pre-made pie shell.

I used the same recipe I used for my strawberry chocolate ganache tart.

Into a cooled tart shell, pour the lemon curd and spread evenly. Top with the cooked blueberries, taking care not to mix it into the lemon curd. Refrigerate until serving.