Preheat the oven to 450°F with a rack in the middle position.

Combine the parsley, mint, scallions, and onion in a large bowl and set aside.

Lightly brush both sides of each slice of bread with some of the olive oil and rub all over with the garlic cloves. Place the bread slices directly on the oven rack and toast until browned and crisped, 4 to 6 minutes. Let cool slightly, cut into 1-inch cubes and set aside.

Season both sides of each steak generously with salt and pepper. Heat a large cast-iron skillet over medium-high heat. Add 1 tablespoon of the olive oil and swirl to coat the skillet. Add the steaks and cook until seared on both sides and still pink in the middle, about 5 minutes per side. Transfer the steaks to a plate and let rest 5 to 10 minutes. Thinly slice and loosely cover with foil.

Whisk the remaining olive oil with the lemon juice and season with salt and pepper to taste. Drizzle the dressing over the herb mixture and gently toss to coat. Add the bread cubes, the sliced steak, and 1 cup of the blackberries. Toss gently to mix and season with salt and pepper to taste. Transfer to a large platter and top with the remaining berries. Serve immediately.