Remove and discard stems. Gently wash; put aside to dry while preparing filling.
In a bowl, beat cream cheese until smooth. Add spring onions, mayonnaise, lemon juice, pepper; mix well.
Stir in crabmeat, tomatoes, and cheese. Spoon mixture evenly into mushroom caps; sprinkle each cap with 1 tablespoon breadcrumbs, and place on a baking sheet. Bake for 15 minutes or until tops are lightly.