Preheat the oven to 350°F (175°C) and lightly butter a 9-by-13-inch baking dish.

Melt 4 tablespoons of butter in a large skillet over medium heat. Add the sliced onions, salt, and black pepper.

Cook the onions, stirring every 10 minutes, for 40–50 minutes until deeply golden and caramelized.

While the onions cook, heat the heavy cream and whole milk in a medium saucepan over medium heat until steaming but not boiling.

Whisk together the flour, garlic powder, and cayenne pepper in a small bowl, then slowly whisk the mixture into the hot cream. Cook 2 minutes until slightly thickened.

Reduce heat to low, add shredded cheddar, and stir until the cheese is fully melted and the sauce is smooth.

Transfer the caramelized onions to the prepared baking dish and pour the cheese sauce over them, folding gently to coat.

In a separate bowl, combine panko breadcrumbs with melted butter until evenly moistened, then sprinkle over the onion mixture.

Bake for 20–25 minutes until the topping is golden brown and the sauce bubbles around the edges.

Garnish with chopped parsley if desired, and serve hot.