Peel, wash and weigh the onions. Or do not weigh, and take the approximate weight "by eye".
Cut into thin half-rings or, if the bulbs are large, quarters of rings.
Pour into a saucepan one, a maximum of two tablespoons of vegetable oil (extra fat we do not need in the dish), spread the onion and passeruem until half-cooked and transparent, WITHOUT closing the saucepan lid. From time to time stir. Don't fry! Here is roughly such he should make. You can put out and under the lid, but then all the onion juice will remain, and the finished cake can flow.
Until the onions passerbys, take the form (I glass 27 cm in diameter, rim height of 3.5 cm), line it up with parchment paper and spread it spasserovannye bow. The photo shows that the edge of the rim is still about 1 cm of free volume.
Prepare the filling: eggs, two types of flour and non-acute mustard lightly whisk with a small amount of milk, as soon as the mixture is uniformly mixed, add the remaining milk, salt to taste (mustard is already salty, do not forget about it). Again, mix everything uniformly and pour the liquid mixture into the form on the onion. Now the form is filled almost to the brim. The surface is leveled (in the photo the surface without leveling) and put in a preheated 180° oven for 1 hour. In 30-40 minutes you look, it's not whether the top. Personally, I at this point reduce the temperature to 170° and the remaining time bake with it.
After an hour, turn off the oven, put on the chef's gloves and take out the form with the cake. The oven is closed, so as not to cool down. Here's a pie on top.
We take a heat-resistant dish, cover the form with it and turn the cake on it with a confident movement. Here is such he now already upside down on platter.
And again are posing on this same platter in TURNED off hot the oven minutes on 30. During this time, the top of the cake will grab, and the onion juice will flow down, evenly distributed over all layers.
After a specified period of time, remove, cool slightly, cut and eat. That's the cut. Bon appetit!