Place 2 tablespoons of olive oil in large pot over medium heat. Add the onion, bell pepper, mushrooms, celery (if using), and garlic. Stir to combine, reduce the heat to low and cover with a lid. Let the mixture cook over low heat for 30 minutes. Stir in the salt and Italian seasoning blend. Transfer the vegetable mixture to a bowl and set aside.
Add the remaining 1 tablespoon of olive oil to the now empty pot. Add the ground beef and pepperoni. Cook until the ground beef is browned and break up the meat using the back of a spoon. Add the vegetable mixture, chicken stock and crushed tomatoes. Stir to combine and bring to a simmer. Serve with freshly ground black pepper.