Prepare outdoor grill with hot coals or heat oven broiler.
Whisk mustard, juice, chili, salt, cumin and pepper in bowl. Whisk in oil, until thick. Cover and refrigerate until ready to use.
Brush both sides of steak with 2 tablespoons dressing.
Grill or broil steak 4 minutes. Turn over; cook 3 to 4 minutes, until 135 degrees F for medium-rare. Remove to platter; let stand 10 minutes. Thinly slice against grain.
Mix lettuce, onion, avocado, peppers and steak with dressing.
Top each tortilla with 1/2 cup salad, then salsa and sour cream. Makes servings.