Pull bones out of salmon with pliers and cut into chunks, then poach in water.
Saute onion, pepper and mushrooms in butter until tender.
Add tomatoes and cooked salmon.
Drain pineapple, reserve syrup.
Combine sugar, cornstarch and mustard. Stir in with the pineapple syrup, vinegar and soy sauce. Add to onion mixture. Cook, stirring constantly until thick and clear. If needed, may be thinned with water.