In a small, dry skillet over medium heat, toast the cumin and coriander, shaking often, until fragrant, about 1 min. Let cool. Combine with the peppercorns and chile flakes; grind briefly in a spice grinder (or pound in a mortar until still a little coarse). Combine the spices with the oil and spread evenly all over the steak. Wrap well and refrigerate for at least 2 hours or overnight.
In a food processor, combine the Roquefort, butter, shallot, thyme, tarragon, salt, and pepper. Process until smooth and creamy. Add the walnuts and process again briefly, so that the walnuts remain chunky. With a spatula, scrape the mixture onto a sheet of waxed paper or parchment and roll into a log about 1-1/2 inches in diameter. Chill until ready to use.
Heat the broiler. Broil the steak until rare to medium rare, about 8 min. on each side, or until a meat thermometer in the thickest part of the meat registers 115º to 125°F. Remove from the broiler and allow the meat to rest for 5 to 10 min.
Cut the butter roll into thin slices. Using a very sharp knife, slice the meat across the grain into the thinnest possible slices. Arrange the steak on four heated plates and top with the butter slices. Run each plate under the broiler just long enough to melt the butter. Sprinkle with chives, if using, and serve immediately.