Chop up the three onions and sweat them with the oil in a pan until they are translucent and then let them cool.
The five eggs are beaten vigorously with a mixer. Then you add the milk. Finally, add the cheese, salt, baking powder, corn flour, the corn with the liquid and the fried onions to the egg-milk mixture.
The whole thing is then filled into a tart pan (the mixture should be about 2 1/2 cm high in the mold) and baked for about 30 minutes (until the dough loosens from the edges and the dough is golden brown) at 250 degrees.