Preheat your grill or a skillet over medium-high heat.

In a small bowl, combine the cracked mustard seeds, key lime zest, olive oil, salt, and pepper to create a paste.

Pat the top sirloin steak dry with paper towels and season both sides with salt and pepper.

Rub the mustard seed paste all over the steak, ensuring an even coating.

Place the steak on the preheated grill or skillet. Cook for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness. Use a meat thermometer to check the internal temperature (130°F for medium-rare).

While the steak is cooking, prepare the horseradish-parsley butter. In a bowl, mix the softened butter, prepared horseradish, chopped parsley, salt, and pepper until well combined.

Once the steak is cooked to your liking, remove it from the grill or skillet and let it rest for 5 minutes.

Slice the steak against the grain and serve topped with a dollop of horseradish-parsley butter.