Place steaks on a cutting board and season with salt and pepper on all sides.
Place the seasoned steaks on the hot grill, and cook for 5-6 minutes on each side for medium-rare (1-inch steak) or 6-8 minutes on each side for a 1½ inch steak.
For a perfect medium rare, your instant read meat thermometer should read 135°F.
Remove steaks from the grill, top with butter and let rest for 5 to 10 minutes before serving.
While steaks are resting, make the salsa verde. In a small food processor pulse the parsley, garlic cloves, salt, pepper, capers and anchovies until combined, but still able to see the ingredients clearly (slightly chunky). Add the parsley mixture to a small bowl and add the olive oil, red wine vinegar, red pepper flakes, lemon juice and lemon zest. Stir until combined.
Slice the steak against the grain and serve with salsa verde.