Step 1) Combine marinade mix, oil and water in a large resealable plastic bag. Slice each mushroom cap into 3 slices; add steak and mushrooms to marinade. Marinate in the refrigerator for at least 30 minutes for flavors to develop.
Step 2) Combine Greek yogurt, buttermilk, dill weed, garlic powder, onion powder, black pepper and blue cheese in a small bowl. Mix well; chill until ready to serve
Step 3) Heat grill to medium-high heat.
Step 4) Remove steak and mushrooms from marinade; discard marinade. Grill steak over Direct Medium heat for 7 to 15 minutes, turning once, for medium-rare or to desired doneness. Grill mushrooms for about 5 to 7 minutes, turning once. Remove mushrooms from grill; cover and keep warm while finishing steak. Remove and cover steak with foil when grilled to desired doneness.
Step 5) Brush cut edges of romaine with 1 teaspoon oil and place cutside on grill. Grill 1 to 2 minutes over Direct Medium heat, or until slightly charred. Remove from grill.
To serve, slice steak into thin slices. Portion steak and mushrooms onto each romaine half. Drizzle with 2 tablespoons Blue Cheese Dressing. Top with tomatoes. Serve immediately.