How to Make It
Preheat oven to 450°.
Heat a large Dutch oven over medium-high heat. Add butter and oil; cook 1 minute or until butter is foamy. Add onion, leek, and thyme; cook 5 minutes, stirring frequently. Sprinkle with salt; cover, reduce heat to medium-low, and cook 15 minutes or until onions are translucent, stirring occasionally (do not allow onion to brown). Add wine; bring to a boil. Cook 5 minutes or until liquid almost evaporates. Combine milk and flour in a small bowl, stirring with a whisk. Add flour mixture, pepper, and nutmeg to pan; return to a boil. Simmer 5 minutes or until mixture thickens.
Spoon onion mixture into a 2-quart glass or ceramic baking dish coated with cooking spray. Top with panko. Coat panko with cooking spray. Bake at 450° for 10 minutes or until bubbly and top is golden brown. Let stand 5 minutes before serving.