Preheat oven to 350 F.
In a skillet cook the onions until translucent, add salt and pepper
In a bowl, whisk the eggs until they become foamy.
In a separate bowl, add the corn flour, then the cooked onion and the milk, and mix.
Add the eggs and the cheese grated. Mix well.
Grease a baking dish with oil and pour the mixture in it.
Bake for 30 to 40 minutes or until the surface is golden brown. Poke the sopa paraguaya with a toothpick, it should come out clean.
Let cool and cut into individual pieces.