Soften gelatin in water in top of a double boiler 5 minutes; cook over simmering water, stirring until gelatin dissolves.
Cook spinach according to package directions, omitting salt; drain well.
Combine dissolved gelatin, spinach, mayonnaise, horseradish, onion, lemon juice, salt, pepper, and hot sauce in container of an electric blender; process until smooth.
Spoon mixture into a lightly oiled 1 1/2-cup mold or bowl. Chill overnight. Unmold onto a serving platter. Spread sour cream over mold. Serve with crackers.