In 4-quart Dutch oven, melt 1/4 cup butter over medium heat. Cook onion in butter 3 minutes or until tender. Stir in chopped chard stalks; cook 12 to 14 minutes, stirring occasionally, until tender and beginning to brown. Increase heat to medium-high. Add chopped chard leaves, salt and pepper; cook 4 minutes or until wilted. Drain well, pressing with back of spoon. Spread mixture in casserole.