Basil, tomatoes, and olive oil refine this sandwich. The soft bread soaks up the juices of fresh-off-the-grill steak, so no flavor is lost.
Let steaks stand at room temperature for 30 minutes. Heat grill to high. Brush grates with oil. Season steaks with salt and pepper. Grill for 5 minutes per side for medium-rare. Let rest for 10 minutes. Thinly slice steaks across the grain.
Meanwhile, drizzle tomatoes with oil; season with salt and pepper. Brush bread with oil. Grill tomatoes until lightly charred and soft, 3 to 4 minutes per side. Grill cut sides of bread until lightly charred, 30 seconds to 2 minutes.
Layer sliced steak and grilled tomatoes over bottom halves of bread. Garnish with basil and romaine leaves, drizzle with oil, and season with salt and pepper. Sandwich with top halves of bread. Cut each loaf crosswise into 6 sandwiches.