In a large bowl, dissolve the Jell-O in the boiling water, stirring for a minute. Add the tomatoes, onion, tomato puree, and tomato juice and mix until the aspic is well combined. Pour the mixture into a lightly greased 1-quart mold or 8-inch square glass dish. Refrigerate, lightly covered, overnight. To remove from the mold, briefly* dip it into a bowl of hot water and unmold onto a chilled plate.
To make the sauce, in a small bowl, whisk together the lemon juice, mayonnaise, and pineapple juice. Add the crushed pineapple and stir lightly with a fork. Cover with plastic wrap and serve at room temperature with the aspic.
*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.