Heat oven to 425°F. Spray 8-inch square (2-quart) glass baking dish with nonstick cooking spray. Melt butter in small saucepan over medium-high heat. Add onion; cook 2 to 3 minutes or until softened but not browned, stirring frequently.
Add whipping cream and Parmesan cheese; mix well. Bring to a boil. Reduce heat to medium; cook 2 to 4 minutes or until slightly thickened, stirring occasionally.
Arrange polenta slices, overlapping and layering if necessary, in bottom of sprayed baking dish. Top with ham and Swiss cheese. Pour sauce over top.
Bake at 425°F. for 20 to 25 minutes or until sauce is bubbly and cheese is lightly browned. Sprinkle with parsley.