Legend has it that a cook for a Paraguayan President (Carlos Antonio Lopez) was trying to make a corn soup. He accidentally put too much flour in and it became a bread.
Heat an oven to 375°F, with a rack in the lower third. Brush a 9-by-5-by-3-inch loaf pan well with 1 tablespoon of the olive oil and coat with ¼ cup of the cornmeal.
2. Heat the remaining 1 tablespoon olive oil in a large skillet over low heat. Add the onions and saute until tender and translucent; do not let them brown. Set aside to cool to room temperature.
In a medium bowl, mix together the milk, beaten eggs, salt, pepper, red pepper flakes and cooled onions. Sprinkle one-third of the remaining cornmeal evenly over the bottom of the pan. Scatter one-third of the mozzarella evenly over it. Ladle one-third of the milk, egg and onion mixture over the cheese. Repeat two more times. The mixture will look quite wet.
Set the pan on a baking sheet and bake for about 1 hour, until puffed and golden brown and quite fluffy; do not let it get too firm, or it will be dry. Cool in the pan on a rack.
5. Run a metal spatula around the sides of the pan to loosen the sopa, place a platter or tray over the top and invert to unmold.