How to Make It
In a large saucepan, cover the potatoes with lightly salted water and bring to a boil. Cook the potatoes over high heat until tender, about 20 minutes. Drain and let cool, then quarter. Meanwhile, in a medium saucepan of boiling salted water, cook the green beans until crisp-tender, 5 to 6 minutes. Transfer the beans to a colander and refresh under cold water. Drain and pat dry.
Preheat the broiler or light a grill. Season the strip steaks with salt and pepper and broil them about 8 inches from the heat source or grill directly over the fire for 10 to 12 minutes for medium-rare meat. Allow the steaks to rest for about 5 minutes, then slice crosswise 1/4-inch thick.
In a glass jar, combine the sherry vinegar, mustard and oil. Cover the jar tightly with a lid and shake until the vinaigrette is emulsified.
In a large bowl, toss the potatoes and beans with the lettuce, tomatoes, radishes and carrots. Pour two-thirds of the vinaigrette over the vegetables, season with salt and pepper and toss to coat. Add the remaining vinaigrette and the avocado, toss gently and mound the salad on plates. Arrange the steak over the salad and serve.