Peel and chop the onions as finely as possible. Heat the oil in a frying pan and cook over very low heat for a minimum of 20 minutes. We pick up when you are translucent and paleda (don’t let it brown). Let it cold down.
We cover the base and sides of a 22 cm removable mold with the shortcrust pastry. We place parchment paper on top and fill with some weight, we have some chickpeas that we use only for this purpose. we cook in the bottom of the oven, preheated to 190 ºC with heat above and below, for 20 minutes. We withdraw and reserve.
Beat the eggs with a whisk until double your 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) over the cooked shortcrust pastry and fill with the previous mixture. We put it back in the oven, preheated to 190 ºC with heat above and below, and cook for 30 minutes. we left rest the cake about 60 minutes before serving so it sets up a bit and doesn’t fall apart when cutting.