In a medium bowl, combine the softened butter, chopped parsley, lemon zest, lemon juice, and a pinch of salt. Mix well until all ingredients are thoroughly combined.

Place the mixture onto a piece of parchment paper or plastic wrap and roll it into a log shape. Twist the ends to secure and refrigerate until firm, about 30 minutes.

Preheat your grill or cast-iron skillet over medium-high heat.

Pat the Porterhouse steak dry with paper towels to remove excess moisture. Season both sides generously with salt and freshly ground black pepper.

In a small bowl, mix together the celery seeds and lemon zest. Press this mixture onto both sides of the steak to create a crust.

Drizzle the olive oil on the grill or in the skillet. Place the Porterhouse steak on the grill or skillet and cook for about 4-5 minutes on one side, without moving it, to achieve a good sear.

Flip the steak and continue cooking for another 4-5 minutes for medium-rare, or until desired doneness. Use a meat thermometer for accuracy (130°F for medium-rare).

Once cooked to your liking, remove the steak from the heat and let it rest for 5-10 minutes to allow the juices to redistribute.

Slice the steak and serve with a pat of the parsley-lemon compound butter on top.