1Spray two 10-ounce oval or round individual casseroles with cooking spray. Heat oil in 10-inch nonstick skillet over medium-high heat. Cook onion in oil about 2 minutes, stirring occasionally, until tender.
2Mix onion, bread, cheese and chicken; spoon into casseroles. Beat remaining ingredients with hand beater until blended; pour over bread mixture. Cover and refrigerate at least 2 hours but no longer than 24 hours.
3Heat oven to 325°F. Bake uncovered 30 to 35 minutes or until knife inserted in center comes out clean. Let stand 5 minutes before serving.