Start by slicing the white onions into rounds.

Take a pot and melt the butter and olive oil over medium heat. Once melted, introduce the sliced onions and cook them covered on low heat for approximately 15 minutes until they become caramelized. Remember to stir occasionally.

Mix the cornstarch with the beef broth, then pour this mixture over the caramelized onions. Let everything simmer together for an additional 10 minutes, maintaining medium heat.

To construct the soup, carefully ladle a portion of the prepared mixture into oven-safe bowls. Arrange 2-3 slices of bread on top of the soup and cover them with a tablespoon of the blended cheeses.

Place the soup-filled bowls in the oven and broil them for around 5 minutes, or until the cheese is melted and achieves a slight golden hue.