"This is one of my kids' favorite meals, and it's incredibly simple and quick. You can add a little beef bouillon for a beefier taste. Serve with French bread or breadsticks. Dip bread directly into the pot or ladle into bowls to serve."
Heat olive oil in a skillet over medium-high heat. Saute onion in hot oil until soft and translucent, 4 to 6 minutes.
Heat a large pot over medium-high heat. Cook and stir beef in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease.
Stir beef, onion, and pizza sauce together in the same pot over medium-high heat. Add mozzarella cheese and Cheddar cheese one handful at a time, stirring until cheeses melt into the fondue before adding the next handfuls. Ladle fondue into bowls.