Heat oven to 350°F. In small bowl, mix mayonnaise and dressing until well blended.
In ungreased 9-inch pie pan, layer sauerkraut, half of the cheese, the corned beef and mayonnaise mixture. To remaining half of cheese in bag, add onions; toss to mix. Sprinkle over top.
Bake 20 to 25 minutes or until bubbly around edge. Serve spread with crackers.