Peel and chop the onions as fine as possible. Heat the oil in a pan and fry over a very low heat for a minimum of 20 minutes. We withdraw when it is translucent and paleda (don’t let it brown). Let it cold down.
Cover the base and sides of a 22 cm mold with the broken dough. We place vegetable paper on top and fill with some weight, we have some chickpeas that we use only for this purpose. We cook in the lower part of the oven, preheated to 190 ºC with heat up and down, for 20 minutes. We withdraw and reserve.
We beat the eggs with some rods until double its volume. Add the evaporated milk, season to taste and stir gently. Add the poached onion and grated Manchego cheese. We mix carefully.
Spread a thin layer of mustard (optional) on the cooked shortcrust pastry and fill with the previous mixture. Put it back in the oven, preheated to 190 ºC with heat up and down, and cook for 30 minutes. We let stand the cake about 60 minutes before serving so it sets a bit and doesn’t fall apart when sliced.