Drain chicken and place in medium size bowl, add mayo, celery, onion and mix well. Toast the bread. Spread two tablespoons of chicken mixture on each slice of toasted bread. Put your oven on to broil. Place the oven rack on the highest shelf. Take a cookie sheet covered with tin foil (for easy clean up)and put your open sandwiches on the cookie sheet. Put about a 1/2 cup of shredded cheese on each. Place in oven for two minutes maximum. I stand by the oven and watch . The cheese melts fast and it burns even faster. You want to melt the cheese which warms the chicken. Serve with or with out silverware we pick it up with our hands.
Lunch can get so boring. My husband asked me for a warm lunch. I usually make this with tuna for myself and 5 year old son, but my husband is allergic to fish. So I substituted it with chicken. It was a hit. It's very gratifying to feed my family food that tastes great and makes them feel good.