1. Preheat oven to 350°.
2. Melt 1 1/2 teaspoons butter in a large skillet over medium-high heat. Add onion and 1/8 teaspoon salt to pan; cook 15 minutes or until tender, stirring occasionally. Add garlic; cook 10 minutes or until onion is golden brown and very tender, stirring occasionally. Remove from heat; cool 5 minutes.
3. Combine the remaining 1/8 teaspoon salt, mayonnaise, and next 4 ingredients (through Worcestershire sauce) in a medium bowl, stirring until smooth. Stir in onion mixture. Transfer mixture to a 1-quart baking dish coated with cooking spray.
4. Melt remaining 1 1/2 teaspoons butter. Combine melted butter and panko. Sprinkle panko mixture evenly over onion mixture. Bake at 350° for 15 minutes or until thoroughly heated and bubbly. Sprinkle with parsley. Serve hot.