Place the potatoes in a large saucepan and add enough cold water to cover; bring to a boil. When the water comes to a boil, salt the water, then cook the potatoes until they are just fork tender, 6-8 minutes.
Place all the peppercorns on a work surface. Using a small, heavy skillet, whack the peppercorns, then roll the skillet over the peppercorns to coarsely crush. Sprinkle the steaks with the kosher salt and coat with the crushed peppercorns.
Meanwhile, heat a cast iron skillet over medium-high. Add the oil, one turn of the pan. Add the steaks and cook until browned, turning once, about 6 minutes; transfer to a plate and cover to keep warm. Reduce the heat to medium and melt 2 tbsp. butter in the skillet. Add the shallots and garlic; season with salt and cook, stirring often, until softened - about 2 minutes. Add the cognac (off heat) and swirl to blend. Add the stock and bring to a bubble. Add the cream and simmer until thickened, 3-5 minutes.
Drain the potatoes; return to the hot pot. Add 2 tbsp. butter and the herbs. Season with salt; toss to coat.
Arrange the steaks on plates; top with the sauce and the arugula (optional). Serve each steak w/potatoes.