Preheat oven to 375 degrees F. In a large saucepan, cook noodles, mushrooms, stir-fry vegetables, and onion in a large amount of boiling water for 3 minutes. Drain; return to saucepan.
In a medium bowl, stir together cream of mushroom soup, milk, dill, and salt. Stir into noodle mixture. Fold in tuna. Spoon mixture into a 1-1/2- to 2-quart oval or rectangular baking dish. Bake, covered, for 25 minutes.
Uncover; sprinkle Parmesan cheese over tuna mixture. Bake, uncovered, about 5 minutes more or until heated through. Makes 4 (1-cup) servings.