In a shallow dish, whisk together oil, vinegar, thyme, garlic, and salt and pepper. Add the steaks and turn to coat both sides well. Let stand for 10 minutes. Heat cast iron skillet over high heat and add the steaks. Brown both sides well and place skillet in 400°F (200°C) oven for about 8 minutes or until medium rare or desired doneness. Let stand 3 minutes. Cut each steak in half (if using 2 larger steaks) or thin strips to serve.
Blue Cheese Sauce: Meanwhile, in a small skillet, heat butter over medium heat and sauté the shallot and thyme for 2 minutes or until softened. Stir in the walnuts (if using) and sauté for 1 minute to toast. Add cheese, reduce heat to low, and cook, stirring occasionally until melted. Stir in parsley and spoon over steaks to serve.
This classic, one-pan meal is perfect for those nights when you want to treat your family to something delicious and hearty without the big cleanup. For added creaminess, add a generous dollop of sour cream!