1. Preheat oven to 350°.
2. Melt butter in a large skillet over medium-high heat. Add onion to pan; sauté 2 minutes. Add mushrooms; sauté 5 minutes. Add wine. Bring to a boil; boil, uncovered, 3 minutes or until liquid almost evaporates.
3. Sprinkle flour over mushrooms. Cook, stirring constantly, 1 minute. Stir in stock and next 3 ingredients (through pepper). Bring to a boil; reduce heat, and simmer, uncovered, 7 minutes or until mixture thickens, stirring occasionally. Remove from heat, and let stand 3 minutes.
4. Stir in cheddar cheese, mayonnaise, and lemon juice. Stir in broccoli and chicken. Spoon mixture into a 13 x 9-inch glass or ceramic baking dish coated with cooking spray.
5. Sprinkle 1 tablespoon Parmesan cheese over top of casserole. Sprinkle panko over cheese; coat panko with cooking spray. Sprinkle 2 tablespoons Parmesan cheese over panko.
6. Bake, uncovered, at 350° for 20 to 22 minutes or until bubbly and browned. Let stand 10 minutes before serving.