Cook the pasta shells in a large pan of boiling water until just tender (al dente). Drain and rinse well.
Measure the butter into a microwave proof bowl and heat on full power for 1 minute.
Stir in the mushrooms and cook on full power for 1 minute, then stir in flour and gradually mix in the milk. Heat on full power for 6 mins, stirring once during cooking and again at the end of the cooking time.
Stir in the seasoning, tuna, eggs and cooked pasta shells and toss lightly until evenly coated with sauce.
Turn into a flameproof dish, sprinkle with the grated cheese and heat on full power for 10 mins until heated through, then brown under a pre-heated grill until cheese is golden.
Sprinkle with parsley and serve with fresh tomato salad.