Preheat oven to 350 degree F. In a medium saucepan cook onion and garlic in hot oil until tender. In same saucepan with the onions and garlic, prepare rice pilaf mix according to package directions.
Meanwhile, drain artichoke hearts, reserving marinade. Coarsely chop artichokes; set aside. Cut off mushroom stems even with caps; discard stems. Remove gills, if desired. Brush mushrooms with some of the reserved marinade; discard any remaining marinade. Place mushroom caps, stemmed sides up, in an ungreased shallow baking pan.
Bake, uncovered, for 15 to 20 minutes or until mushrooms are tender. Transfer to individual plates, stemmed sides up.Stir artichoke hearts and cheese into hot rice pilaf mixture; spoon into baked mushroom caps. Makes 6 main-dish servings.