Place the softened butter into the bowl of a stand mixer with the paddle attachment (if using a hand-held mixer, place butter into a mixing bowl) and beat until light and fluffy, 3 to 4 minutes. Add chives, mustard, salt and pepper. Beat on medium speed until everything is well mixed, 1 to 2 minutes.
Crumble in the Castello Extra Creamy Danish Blue cheese and gently mix until it’s incorporated. It shouldn’t be perfectly smooth, there should be a few small flecks of cheese here or there. Taste for seasoning and add salt and pepper, if desired.
Place the butter in the center of a sheet of parchment paper, wax paper or plastic wrap. Form a log by rolling the bottom half of the paper over the butter (roll it back and forth a few times until it’s the thickness you want), then roll it all the way closed. Twist the ends to seal and tighten the log (you can tie them with twine if you like) and refrigerate until hardened, about two hours.
This recipe makes about 12 servings of the butter. The sealed roll will keep for 4 to 5 days in the refrigerator or up to 3 months frozen.
Prepare a charcoal or gas grill. If gas, preheat on high for at least 15 minutes ahead of time. Remove steaks from the refrigerator 30 minutes before cooking so they come to room temperature. Season them liberally with course salt and pepper on both sides.
Rub grill grates well with oil. Grill steaks until medium rare, about 3 to 4 minutes per side (or however you like them done).
Remove steaks and immediately top each with a slice of blue cheese compound butter. Let it melt for 5 to 7 minutes as the steaks rest. Serve with Balsamic Roasted Red Onions with Thyme.