Stir together cayenne pepper, paprika, black pepper, cumin, onion powder, garlic powder, allspice, thyme, and 1/4 cup olive oil in a large bowl until evenly mixed. Smear the flat iron steaks with this paste, then wrap with plastic wrap, or place into a resealable plastic bag. Marinate in refrigerator for 6 hours.
Prepare avocado salad by gently mixing cubed avocado, green onion, red bell pepper, red onion, cilantro, and tomatoes. Drizzle with lime juice and olive oil; season to taste with salt and pepper. Cover and refrigerate until ready to use.
Preheat an outdoor grill for medium-high heat and lightly oil grate.
Grill flat iron steaks until cooked to desired degree of doneness, or to an internal temperature of 135 degrees F (57 degrees C) for medium-rare, about 4 minutes per side. Remove from grill and allow to rest for 5 to 10 minutes. While the steaks are resting, cut each baguette into three pieces, then halve the pieces horizontally. Toast bread, cut-side down on grill until golden brown. Turn baguettes over, and place sliced cheese on the cut sides; allow cheese to melt.
To assemble sandwiches, thinly slice the flat iron steaks across the grain, and pile onto baguettes. Top with avocado salad before serving.