Heat oven to 350°F. Melt butter in large nonstick skillet over medium heat. Add onion; cook 8 to 10 minutes or until very tender, stirring frequently.
Meanwhile, place cheese in ungreased decorative ovenproof shallow baking dish or plate. Bake at 350°F. for 10 to 12 minutes or until cheese is soft.
Add cranberries, brown sugar, salt and sherry to onion; mix well. Cook 5 minutes or until sugar is dissolved, stirring frequently. Stir in marjoram; cook and stir 1 minute.
To serve, spoon warm onion mixture over cheese. Serve with assorted crackers and small cocktail bread slices. Store in refrigerator.