In a bowl, combine the vinegar, fresh lemon juice and Dijon mustard. Gradually whisk in the oil. Season the salad dressing to taste with salt and pepper. (Can be prepared 1 day ahead- cover and refrigerate.)
ASSEMBLE THE SALAD:
Bring a pot of salted water to boil. Add the green beans and cook until just tender, about 4 minutes. Drain and refresh with cold water, then drain again. (Can be prepared 1 day ahead. Cover and refrigerate.)
Preheat the oven to 400°F. Place the bread slices on a baking sheet and bake until golden, about 5 minutes. Top the bread with cheese. Bake until the cheese softens and is heated through, about 10 minutes.
Meanwhile line the plates with alternating Boston and red lettuce leaves. Mound the green beans in the center of the leaves.
Cut the avocado into thin slices and fan around the beans. Place the cheese croutes on one side of the salad. Garnish with tomato wedges. Drizzle with vinaigrette and sprinkle with walnuts.