In a Dutch oven, place potatoes and add enough oil to cover by an inch. Be sure the oil line is at least 2 inches from the top edge of pot. Cook potatoes over medium-high to high heat and bring oil to a boil. Cook 20-30 minutes, stirring frequently, or until frites are deep gold. Remove with a slotted spoon to a cooling rack set over a baking sheet to drain. Sprinkle with flaky sea salt to taste.
While the potatoes are cooking, season the steak liberally with salt and pepper. Heat a cast-iron skillet over medium-high heat. Rub skillet with oil and drizzle some over steaks. Cook steaks to deep and crusty, and browned on both sides, about 8 minutes for rare and 10 minutes for medium. Let steaks rest.
Add butter to the skillet and melt; add shallots and stir 2 minutes. Add flour, cook for 1 minute then add brandy or sherry and stock, and bring to bubble. Stir in Worcestershire and tomato paste, and season with salt and pepper.
Serve the steaks with small pile of cress and frites alongside. Top steak with lots of sauce, passing mustard at table.