Thoroughly wash the collard greens. Strip the leaves from the thick central ribs. Stack several leaves on top of one another, roll them tightly like a cigar, and slice them into thin ribbons (a chiffonade). Set the greens aside.

Preheat a large Dutch oven or deep skillet over medium heat and add the 2 tablespoons of olive oil. Add the diced yellow onion and sauté for 5 to 7 minutes until softened and translucent. Add the minced garlic and sauté for 1 minute until fragrant. Do not allow the garlic to brown.

Add the prepared collard greens to the pan in batches, stirring until they wilt down slightly before adding the next batch. Pour in the broth, smoked paprika, and red pepper flakes (if using). Cover the pan, reduce the heat to low, and simmer the greens for 15 to 20 minutes, or until tender.

Once cooked, drain the collards well in a colander to remove excess liquid, ensuring they are as dry as possible. Press down on the greens to squeeze out moisture. Season with salt and pepper, then transfer the greens to a large mixing bowl.

While the greens are simmering, prepare the sauce. Melt the 4 tablespoons of butter in a medium saucepan over medium heat. Whisk in the flour to create a smooth paste (a roux). Cook the roux, stirring constantly, for 1 to 2 minutes to cook out the raw flour flavor. Slowly whisk the warm milk into the roux, pouring in a steady stream to prevent lumps.

Bring the sauce to a gentle simmer, whisking frequently until it thickens sufficiently to coat the back of a spoon (about 5 to 8 minutes). Reduce the heat to low. Remove the saucepan from the heat, and stir in the nutmeg and dry mustard powder.

Add the Gruyère and 1 cup of the sharp cheddar cheese. Stir until the cheeses are completely melted and the sauce is smooth. Taste the sauce and adjust seasonings with salt and pepper as needed. Pour about three-quarters of the cheese sauce over the drained collard greens in the mixing bowl and stir thoroughly to coat every piece of the greens evenly.

Preheat the oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Spread the cheesy collard green mixture evenly into the prepared baking dish. Pour the remaining cheese sauce over the top of the greens.

In a small bowl, combine the panko breadcrumbs, 2 tablespoons of melted butter, the reserved 1/2 cup of sharp cheddar, and the grated Parmesan cheese. Toss the mixture until the breadcrumbs are lightly coated in the butter and cheese. Sprinkle this topping mixture evenly over the entire surface of the collard greens.

Place the gratin in the preheated oven and bake for 25 to 30 minutes, or until the sauce is bubbly around the edges and the breadcrumb topping is golden brown and crisp. Remove the gratin from the oven and allow it to rest for 10 to 15 minutes before serving. This resting time helps the gratin set up, making it easier to scoop and serve.