Remove the duck breast from the packaging and dry on paper towels to remove all excess moisture.
With a sharp knife, lightly score the skin in a crosshatch formation. (This will keep the breast from puckering up when seared).
Season both sides generously with salt and pepper.
Heat a large sauté pan over moderate heat and place the duck breast skin-side down into the pan. Do not touch the breast and allow it to cook until a golden brown color is achieved for about 4-6 minutes.
Reduce the heat to low and cook the duck breast on the second side and add the butter. Gently spoon the browned butter over the breast and cook for 7-8 minutes, until the breast is firm when touching but still has some spring.
Remove the duck from the pan and rest for 8-10 minutes while completing the other elements.
Return the heat of the pan to moderate and add the sliced onions. Cook until translucent in the rendered duck fat, about 3-4 minutes.
Add the kale and white wine to the pan and continue cooking for around 2 minutes until the kale is wilted and the wine has been absorbed into the vegetables.
Shut off the heat, and season to taste with salt and pepper.
Slice the duck breast across the grain and serve on a bed of the wilted onions and spinach. Adorn each plate with pomegranate seeds and crumbled Roquefort.