Mix together flour and salt in a bowl to create a tart shell, then cut cold butter into small cubes. Add the butter and olive oil to the dry mixture and stir/kneed until the consistency becomes that of a coarse meal, and butter has completely dissolved.
In a small cup, combine the egg yolk, ice water and stir well together. Add the egg mixture and mix together well.
Dust working surface with a very small amount of flour. Shape the dough into a disk on a foil (dusted slightly with flour).
Refrigerate the dough overnight.
Preheat oven to 400 F.
Place the dough in 3 tart shells, cover with an aluminum foil and bake for 15 minutes. Then remove the foil and bake for another 10 minutes.
While the crust cools down, whisk together eggs, heavy cream, milk in a small bowl. Then, add cheese and salt and mix well.
Divide the egg mixture into 3 equal parts and pour it into each of 3 tart shells.
Slice the tomatoes and arrange on top of each little tart.
Bake these egg tartlets for 20-30 minutes, until tart shell is golden brown and the filling top turns yellow-brownish and starts to rise, or when the tooth pick inserted in the middle of the tart comes out clean.