1. Place cooked shrimp or crab meat and spring onions in a bowl. Add mayonnaise, mix well and chill.
2. Slice the top of the tomatoes, scoop out the pulp and reserve. Keep tomato shells upside down, chill.
3. Just before serving, arrange the lettuce leaves on a shallow salad plate.
4. Fill the tomato cases with seafood and mayonnaise mixture. Garnish with egg slices and lemon. Place on bed of lettuce and serve.