We will mix the flour, butter, beaten egg yolk, tablespoon of salt and 3 tablespoons of cold mineral water. Let it rest for 30 minutes.
After this time, we will roll out the dough and place it in a 22 cm. mold and let it rest again, while we prepare the filling.
We will heat the oven to 200 ° C.
Melt the butter in a saucepan, add the peeled and thinly sliced onions and let them cook over medium heat for 10 to 20 minutes, stirring occasionally so that they do not burn.
We will spread the mixture on the dough.
We will beat the 3 eggs, we will incorporate the liquid cream, we will season it with salt, black pepper and nutmeg. We will pour it over the onions.
We will put the tartera in the oven for 30 or 40 minutes.
We will serve at room temperature
