Take the steaks out of the refrigerator a half hour before cooking so they can come to room temperature before cooking. Salt and pepper the steaks.
Heat one to two tablespoons of oil in a large grill pan on the stove over medium-high heat. Cook to desired doneness, which will depend on the thickness, about four minutes per side for medium-rare. We recommend 3 minutes on indirect heat and the last minute on direct heat. Transfer steaks to carving plate and let rest for five to ten minutes while you make the sauce.
To make the sauce, add the chopped shallots or green onions to the pan where you cooked the steaks; sauté for a minute. Add the mustard seeds and thyme and sauté for a minute and then add white wine and lemon zest and let simmer for a minute to bring all the ingredients together. Add the butter at the very end and let melt.
Slice steak on carving plate; spoon sauce over the top and sprinkle parsley on top as garnish if you have it.