Place a heavy-bottomed skillet or a cast-iron skillet over medium heat with the oil. Season steaks liberally with salt and pepper, and cook to desired doneness, turning once, about 5 minutes per side for medium (for a steak that is 1-inch thick). Let steaks rest for 10 minutes before slicing.
While steaks are cooking, prepare dressing: In a large mixing bowl, whisk together all ingredients for dressing, and adjust seasoning as needed with salt, pepper and more lemon juice.
To serve, toss lettuce, tomatoes, radishes and avocado with prepared dressing, then divide among serving plates. Thinly slice steaks and top each salad.