Soak the potatoes in cold water for 30 minutes. Drain well and pat completely dry.
Heat the oil to 300°F/150°C. Blanch the fries in batches until pale and just tender, about 5 minutes. Transfer to a rack and chill in the freezer until firm.
Season the steaks with salt and pepper. Heat the oil in a stainless-steel skillet over medium-high until shimmering. Sear the steaks until well browned, about 3 minutes per side. Add the butter, garlic, and rosemary, then baste until the steaks reach medium-rare or medium. Transfer to a plate to rest.
In the same skillet, add the shallot, garlic, and peppercorns. Cook briefly until fragrant. Add the beef broth and cognac, scraping up browned bits. Stir in the cream and simmer until slightly thickened.
Increase the oil temperature to 350°F/175°C. Fry the potatoes until crisp and deep golden, 3 to 4 minutes. Drain, season with salt, then toss with garlic, parsley, and Parmesan.
Slice the steak and serve with the fries. Spoon the sauce over the steak and finish with chives.