For the Flank Steak:
In a large measuring cup, combine the oil, Worcestershire sauce, lime juice, garlic, Fajita & Taco Spice, and brown sugar. Stir well to combine then remove 1/2 cup of the marinade and reserve for the vegetables.
Divide the remaining marinade into two 1 gallon plastic bags. Place one flank steak into each bag and allow to marinate in the refrigerator for 2 hours or longer. Allow the meat to come to room temperature before cooking.
When you are ready to cook, preheat a charcoal or gas grill to medium high heat. Cook the flank steaks 4-5 minutes per side for medium rare. Remove the meat from the grill and allow it to rest a few minutes before slicing.
For the Vegetables:
Combine the vegetables and reserved marinade in a gallon plastic bag and allow to marinate 2 hours or longer. Grill the vegetables alongside or after the steak, turning every minute or so to get a nice sear on all sides.
Transfer the vegetables to a cutting board and chop into bite sized pieces.
For the Dressing:
While the meat and vegetables are marinating, prepare the dressing.
In a blender, combine the cilantro, yogurt or sour cream, minced garlic, lime juice, salt and pepper. Blend well. Add the grapeseed oil to the mixture and blend again.
For the Salad:
On a large platter, lay down a bed of romaine lettuce. Add the warm meat and vegetables, and drizzle on a generous portion of dressing. Garnish with cilantro and cheese (optional).