Heat oven to 375 degrees. Mist a 13-by 9-inch baking dish with non-stick spray, position chicken pieces in dish so they are not overlapping, and set dish aside.
Remove half the chard (6 ounces) from the bag (save rest for another use), rinse chard and drain in colander so there is some water clinging to the leaves. Place chard in microwave-safe container with a lid, and micro-cook on full power for 3-4 minutes, stirring at the halfway point. Chard will be wilted and soft. Drain if there is excess water collected on the bottom.
In same container with wilted chard, add the chopped artichoke hearts, cheese, mayonnaise and chopped garlic and onion. Stir to combine well. Spread mixture evenly over tops of chicken breasts, and bake for 35 minutes.
Note: This recipe doubles easily.