2 large onions, finely chopped
1 (10.75 ounce) can condensed cream of chicken soup
1 cup boiling milk
1/2 cup diced celery
2 tablespoons prepared horseradish
1 tablespoon crushed garlic
3 cups shredded Cheddar cheese
1 cup shredded monterey jack cheese
1 1/2 cups shredded Mexican-style cheese (reserve 1/2 cup for salsa)
3 tablespoons mayonnaise
Preheat oven to 350 degrees F (175 degrees C).
Saute onion, celery and horseradish in the microwave 10 minutes, until tender. Stir in melted cheese and cheese; return to a simmer. Reduce heat to low; mix in brown sugar.
Grease and flour 2 sandwich pan liners. Add chicken, celery, horseradish, garlic, Monterey jack cheese, Mexican-style cheese, mayonnaise, onions and celery mixture.
Bake for 45 minutes, or until chicken is cooked through and juices are clear. Serve over hot tortilla chips or corn chips.