Preheat oven to 400 F.
Bring 6 quarts water to boil and add 2 tablespoons salt. Add the manicotti and cook according to package directions. Drain and reserve. Once you have the water working, start the filling.
In a 6 to 8-quart, heavy-bottomed saucepan, heat the olive oil and butter over medium heat. Add the onions, celery, and garlic and sweat over medium heat until the vegetables are translucent and soft but not browned, about 10 to 15 minutes. Add the ground beef and stir into the vegetables. And cook over medium heat and fully cooked, about 10 minutes, stirring regularly.