Dissolve two 3-ounce packages lime-flavored gelatin in 2 cups boiling water.
Stir in 11/2 cups cold water, 3 tablespoons lemon juice, and dash salt.
Chill till partially set.
Fold in one 9 1/4-ounce can tuna, drained and flaked; one 10-ounce package frozen mixed vegetables, cooked and drained; 1/2 cup chopped celery; and 2 tablespoons finely chopped onion.
Turn into 61/2-cup mold; chill till firm.
Unmold; pass mayonnaise.