Cook pasta in a large pot of boiling water, following package directions but omitting salt, until tender, 9 to 11 minutes. Then drain.
Melt butter in a large oven-safe frying pan over medium. Add onion, celery and mushrooms. Cook until mushrooms are brown and celery is tender, 5 to 7 minutes. Sprinkle flour over vegetable mixture. Stir to combine. Gradually add broth, salt, milk and mustard.
When mixture is thickened, stir in pasta, peas, tuna and oil in can. Gently stir to combine, breaking tuna into large chunks. Cook for 2 minutes. Sprinkle evenly with Parmesan, then panko.
Preheat broiler. Broil in centre of oven until top is golden brown, 2 to 4 minutes.
Serve and enjoy, or freeze for later!