Put a large pot of water on to boil. Once it begins boiling add shells and cook according to the package directions.
While the water is heating, cook hamburger and onion over a medium flame until hamburger is browned.
Add taco seasoning, beans, and diced tomatoes. Be sure to add the juice from the diced tomatoes too. Simmer over low heat for for 5 minutes.
Remove from heat aside and let cool while the shells cook. Once the meat mixture is close to room temperature, add 2 cups of cheese and mix well.
In a small bowl, combine tomato sauce and salsa. Spoon 1/2 cup of tomato sauce mixture into each 13 x 9 baking dish.
Preheat oven to 350 degrees.
After the shells have been drained, spoon meat mixture into each shell and place it in a baking dish.
After all the shells have been filled. Spoon 1/2 cup of the tomato sauce mixture over the shells in each baking dish. Sprinkle 3/4 cup cheese over the stuffed shells in each baking dish.
Cover one baking dish with foil and bake at 350 degrees for 25 minutes. Remove foil and bake for another 5 minutes.