I like to serve the steaks with celery sticks and store-bought onion rings.
Pat the steaks dry; season with kosher salt and pepper. Heat a large cast-iron skillet over medium.
Add the walnuts to the skillet. Toast over medium heat until fragrant, about 2 minutes. Transfer the walnuts to a food processor and let cool. Keep the skillet over medium heat for cooking the steaks.
Add the parsley, watercress, shallot, vinegar, capers, and lemon juice to the walnuts in the food processor. Pulse until finely chopped. With the machine running, stream in the EVOO and process until semi-smooth. Season the pesto with salt.
Add the olive oil, one turn of the pan, to the reserved skillet. Add the steaks and rosemary sprigs. Cook the steaks, undisturbed, until a crust forms, about 3 minutes. Turn the steaks and cook for 2 minutes more. Continue cooking, turning occasionally, for about 3 minutes more for medium-rare and 7 minutes more for medium. During the last minute of cooking, add the butter to the skillet and let it melt. Using a large spoon, baste the steaks with the pan juices.
Transfer the steaks to a cutting board. Let rest for 5 minutes. Cut the steaks against the grain into 1/4-inch-thick slices. Divide the steaks among 4 plates. Top with the pesto and blue cheese crumbles.
While I never turn a steak before it develops a reddish-brown crust, I feel that turning it occasionally after that helps it cook more quickly and evenly.