1Heat oven to 350°F. Heat water, 1/2 cup of the soup and the butter to a rapid boil in 2-quart saucepan. Remove from heat. Stir in potatoes and sour cream just until moistened. Let stand about 1 minute or until liquid is absorbed. Whip with fork until smooth.
2Prepare spinach as driected on box; drain well. Stir together spinach, remaining soup and half of the onions. Alternately spoon heaping tablespoonfuls potato mixture and spinach mixture into ungreased 1 1/2-quart casserole. Run table knife through spinach and potatoes to swirl slightly.
3Bake uncovered 20 to 25 minutes or until hot and bubbly. Top with remaining onions. Bake about 5 minutes longer or until onions are golden brown.