Preheat the oven to 410F
In a medium-sized ovenproof bowl, combine the butter, oil, water, sugar, and salt. Place the bowl in the oven for 15 minutes, until the butter is bubbling and starts to brown just around the edges.
When done, remove the bowl from oven (and be careful, since the bowl will be hot and the mixture might sputter a bit), dump in the flour and stir it in quickly, until it comes together and forms a ball which pulls away from the sides of the bowl.
When dough is cool, press it into the tart pans. It will puff up during baking, so make it a very thin layer, or you won't have room for the filling. Prick the bottom of the tart shells a few times each with a fork.
Bake for 15 minutes, or until the dough is golden brown.
While shells are cooling, make the pastry cream. Heat the milk in the saucepan over medium-high heat until warm (100F, or just warm to the touch).
In a stand mixer, mix together the egg yolks and sugar until smooth and thick. Mix in the flour, cornstarch, and salt. Slowly pour in the warm milk into the egg-sugar mixture with the mixer on low speed. When fully combined, transfer the mixture back to the saucepan.
Cook over medium heat, stirring constantly with a whisk so the bottom doesn't scorch. It will quickly start to thicken to the consistency of pudding. When large steamy bubbles start to pop through the liquid, the pastry cream is nearly done. Continue stirring for 1-2 minutes longer, or until all taste of flour is gone. Remove from heat, and add lemon oil.
Spoon the pastry cream into the tart shells, and dot with blueberries. Chill until firm, 4-5 hours. Makes 12 mini tarts.