Heat the oven to 450ºF.
• Trim the root ends from the leeks; cut the leeks in half lengthwise, then crosswise into ½-inch-thick half-moons. Soak the leeks in a bowl of cold water, allowing any dirt to settle at the bottom. Skim the leeks from the surface of the water and pat dry.
• Strip the kale leaves from their stems; coarsely chop the leaves.

In a large pan over medium heat, warm 2 tablespoons oil until hot but not smoking. Add the leeks, season with salt and pepper, and cook, stirring occasionally, until just soft, 5 to 7 minutes. Add the kale and 2 tablespoons water, season with salt and pepper, and cook until wilted, 3 to 4 minutes. Transfer the leeks and kale to a large bowl.
While the leeks and kale cook, start the cheese sauce.

• Finely chop enough garlic to measure 1 tablespoon.
• Using the large holes of a box grater, grate the Gruyère cheese.
• Strip the parsley leaves from their stems; coarsely chop the leaves.

In a small pot over medium heat, melt the butter until foamy. Whisk in the flour, garlic, nutmeg, and as much Urfa biber as you like. Cook, whisking constantly, for 1 to 2 minutes. Slowly whisk in the milk. Bring to a vigorous simmer over medium-high heat, whisking, just until the milk thickens. Immediately remove from the heat. Add the grated Gruyère and half the parsley, whisking to melt the cheese.

Pour the cheese sauce over the leeks and kale and stir to coat. Season to taste with salt and pepper, if needed. Transfer the mixture to a baking dish and spread in an even layer. Sprinkle with the bread crumbs and Parmesan. Bake until lightly browned and bubbling, 15 to 20 minutes. Remove from the oven and let cool for at least 5 minutes before serving.

Sprinkle the gratin with the remaining parsley and serve warm, straight from the gratin dish or spooned onto individual plates.