For the dressing:
2 tsp anchovy paste
2 small cloves garlic, chopped
3/4 cup mayonnaise
3/4 cup sour cream
1/2 cup fresh flat-leaf parsley
3 scallions, white and green parts, roughly chopped
Juice of 1 lemon
1/4 tsp kosher salt
1/4 tsp fresh black pepper
For the salad:
8 oz flank steak or skirt steak
1 red bell pepper, thinly sliced
8 cups shredded romaine lettuce
In a blender, combine all of the dressing ingredients, and blend until smooth. (You can do this several days in advance; store dressing in the refrigerator in a container with an airtight lid.)
Season the steak with salt and pepper, and grill or broil until medium rare. Set aside for 15 minutes, then slice thinly across the grain.
In a mixing bowl, combine the bell pepper and lettuce. Add a few tablespoons of the dressing, and toss well. Transfer the vegetables to a serving plate.
Top the greens with slices of steak, and drizzle on additional dressing, as much or as little as you like.
Serve at room temperature.