In a large bowl, dissolve the Jell-O in the boiling water, stirring for 2 minutes.
Add the tomatoes, onion, tomato puree, and tomato juice. Mix well. Pour the mixture into a lightly greased 1-quart mold or 8-inch square glass dish.
Refrigerate, covered, overnight.
To remove from the mold, briefly dip it inot a bowl of hot water and un-mold onto a chilled plate. Briefly means about 10 seconds. If the aspic doesn't come out of the mold the first time, dip it again for 5 seconds. Have patience!
To make the sauce: in a small bowl, whisk together the lemon juice, mayonnaise, and pineapple juice.
Add canned crushed pineapple and stir lightly with a fork.
Serve at sauce at room temperature with the chilled aspic.