How to Make It
Preheat grill to very high (about 550°F). Rub steak with 1/2 tablespoon oil; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Place on oiled grates; grill, uncovered, to desired degree of doneness, 4 to 5 minutes per side for medium-rare. Transfer steak to a cutting board; let rest 10 minutes. Slice against the grain.
While steak rests, brush focaccia pieces evenly with 1 tablespoon oil; sprinkle with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Grill, uncovered, turning often, until crisp and lightly charred all over, 4 to 6 minutes. Remove from grill; let cool slightly, about 5 minutes. Tear or cut into 1-inch pieces.
Whisk together vinegar, Calabrian chiles, remaining 5 tablespoons oil, remaining 11/4 teaspoons salt, and remaining 1/2 teaspoon black pepper in a small bowl. Place steak, watercress, arugula, tomato, cucumbers, and onion in a large bowl; add 1/2 cup dressing, and toss to coat. Add focaccia pieces; toss to combine. Let salad stand 5 minutes. Season with salt and pepper to taste. Transfer salad to a large serving bowl; drizzle with remaining dressing.