In a medium bowl, combine the softened butter, chopped Kalamata olives, parsley, lemon zest, salt, and pepper. Mix well until all ingredients are fully incorporated.

Place the mixture onto a piece of plastic wrap, shape it into a log, and wrap tightly. Refrigerate until firm, about 30 minutes.

Preheat your grill or a cast-iron skillet over medium-high heat. Rub the flat iron steak with olive oil, then season generously with salt, pepper, toasted fennel seeds, and chopped chives on both sides.

Place the steak on the hot grill or skillet. Cook for about 4-5 minutes on each side for medium-rare, or until desired doneness is achieved. Use a meat thermometer to check (130°F for medium-rare).

Once cooked, remove the steak from the heat and let it rest for about 5-10 minutes.

Slice the rested steak against the grain. Serve topped with slices of the Kalamata olive compound butter, allowing it to melt over the warm steak.