Preheat oven to 350 degrees F. Lightly coat a 2-quart square baking dish with nonstick cooking spray; set aside.
Cook pasta according to package directions; drain. Rinse with cold water; drain again. Set aside.
Meanwhile, in a medium skillet cook ground beef until brown, using a wooden spoon to break up meat as it cooks. Drain off fat. Set meat aside.
In a large nonstick skillet heat oil over medium heat. Add mushrooms, onion, carrot, celery, garlic, and Italian seasoning; cook for 6 to 8 minutes or until tender, stirring frequently. Add diced tomatoes and 1/4 teaspoon salt. Cook and stir for 2 minutes more. Remove from heat; cool slightly.
Spoon the tomato mixture into a blender or food processor; cover and blend or process until nearly smooth. Set aside 3/4 cup of the pureed tomato mixture.
Return the remaining pureed tomato mixture to the skillet. Stir cooked meat into tomato mixture in skillet. Spoon a rounded tablespoon of the meat mixture into each pasta shell.
Arrange filled pasta shells in prepared baking dish. Spoon the reserved 3/4 cup pureed tomato mixture over the shells.
Cover; bake for 20 minutes. Sprinkle with cheese and fresh tomato. Bake about 5 minutes more or until heated through and cheese is melted. If desired, sprinkle with basil.