Preheat your oven to 375°F (190°C). Season the tomahawk steak generously with salt and pepper. Allow it to sit at room temperature for about 30 minutes.

In a bowl, combine the chopped rosemary and panko breadcrumbs. In another bowl, place the flour and season with salt and pepper. In a third bowl, whisk together the eggs and buttermilk.

Dredge the steak in the flour, shaking off any excess. Dip it into the egg and buttermilk mixture, ensuring it is well coated. Finally, press the steak into the rosemary-panko mixture, covering it evenly.

Heat a large oven-safe skillet over medium-high heat and add a drizzle of olive oil. Sear the crusted steak for about 4-5 minutes on each side until golden brown.

Transfer the skillet to the preheated oven and roast the steak for about 20-30 minutes, or until it reaches your desired level of doneness (135°F for medium-rare). Remove from the oven and let it rest for at least 10 minutes before slicing.

While the steak is resting, heat oil in a deep skillet over medium heat. In batches, dip the pearl onions in flour, then in the egg mixture, and finally in panko breadcrumbs. Fry until golden brown and crispy, about 3-4 minutes. Drain on paper towels.

In a small saucepan, melt the butter over medium heat. Continue cooking until it turns golden brown and has a nutty aroma. Add the chopped shallots and cook until soft, about 3-4 minutes.

Pour in the Marsala wine and bring to a simmer. Cook until reduced by half, about 5-7 minutes. Season with salt and pepper to taste.

Slice the steak and arrange it on a platter. Top with crispy pearl onion rings. Drizzle the brown butter and Marsala-shallot reduction over the steak before serving.