Toast bread, trim crusts. Mix butter and mustard, spread over 1 side of each bread slice. Arrange 6 slices bread buttered sides up in ungreased baking dish 11 3/4 x 7 1/2 x 1 3/4 inches.
Heat oven to 350 degrees. Flake salmon, removing skin and bones. Mix salmon, cheese, olives and onion. Spread salmon mixture evenly over bread in baking dish. Cut remaining 6 slices bread diagonally in half, place buttered sides up on salmon mixture. Rinse frozen peas under running cold water to separate, layer on bread. Mix soup and milk, pour evenly on and around sandwiches.
Bake uncovered until hot and bubbly, 25 to 30 minutes. Serve with dill pickles. Makes 6 servings.
