Drain canned mushrooms; reserving
the liquid. Cook the mushrooms and chopped onions in margarine
until tender, about 5 minutes. Stir in flour.
Gradually stir in the reserved
mushroom liquid and milk. Cook, stirring constantly until
thickened. Remove from the heat.
Add the cooked rice, the cooked
fish, chopped pimiento, salt, and pepper.
Pour into a baking dish. Sprinkle
crushed toasted rice cereal squares on top.
Bake for 25 minutes, or until the
sauce is bubbly and the top is slightly browned.