Melt 1 tablespoon of butter in a small saucepan over medium-high heat. Add the garlic and shallot and cook, stirring, until golden brown, about 3 minutes. Add the coriander seed and peppercorns and cook until fragrant, about 30 seconds. Pull the pan from the heat and add the port and wine. Return the pan to the heat and, if cooking over a gas burner, tip the pan to let the alcohol ignite. (If cooking over electric or halogen, just keep cooking the mixture.) Bring the mixture to a boil, reduce the heat and simmer briskly until syrupy, 20 to 25 minutes. Set aside to cool.
Beat the remaning butter in a medium bowl with a handheld electric mixer or whisk until smooth and light. Add the reduced wine mixture, vinegar, parsley and 1 1/2 teaspoons salt and beat until evenly mixed.
Spread a 12-inch-long piece of plastic wrap on a work surface. Mound the butter across the plastic wrap, about 2 inches from edge nearest you. Fold the bottom edge of the plastic wrap over the butter and roll in up to make a 1-inch-wider log. Twist the ends together in opposite directions (like a party favor) and refrigerate until firm.
Prepare an outdoor grill with a high fire.
Brush grill grate lightly with oil. Season the steaks on both sides with salt and black pepper, to taste. Grill 3 to 5 minutes, turning once, until an instant-read thermometer inserted in the meat reads 120°F to 125°F for rare; 125°F to 130°F for medium-rare; or 130°F to 135°F for medium. Transfer the steaks to a cutting board and let rest for 5 minutes. Serve each steak topped with a 3/4-inch-thick slice of the wine butter.
Any leftover butter can be frozen and used with lamb and pork.