In a large pan over medium heat, saute diced onion in olive oil until softened, about ten minutes.
Add ground beef and cook, breaking it apart with a spoon, until no longer pink. Raise the temperature to medium-high if you need to.
Add 1 can of diced tomatoes to the beef and cook a few minutes more. Set aside.
Warm tortillas slightly to make them more pliable, then fill one with a spread of refried beans, a sprinkle of cheese, a spoonful or two of the beef mixture. Roll it up, then place in a large baking dish. Repeat with all the tortillas.
Spoon about half the second can of diced tomatoes on top of the burritos, and sprinkle the rest of the grated cheese over it.
Bake at 375 for 15-20 minutes, until the burritos are soft and bubbly.