Heat 1 tablespoon olive oil in a large nonstick skillet over medium-high heat. Add the bread cubes and cook until crisp and slightly golden, turning as needed, about 5 minutes. Add the garlic and 1/4 teaspoon each salt and pepper and cook, tossing, until the garlic is slightly golden, about 30 seconds. Transfer to a large bowl; wipe out the skillet. Add the radishes, tomatoes, basil, capers and caper brine to the bowl and toss to combine.
Heat 1 tablespoon olive oil in the same skillet over medium-high heat. Season the steak with salt and pepper, add to the pan and sear until browned on the bottom, about 5 minutes. Turn and sear until browned on the other side, 3 to 5 more minutes. Transfer the steak to a cutting board and let rest 5 minutes.
Add the arugula and the remaining 2 tablespoons olive oil to the bowl with the salad and toss to coat; season with salt and pepper. Thinly slice the steak against the grain and divide among plates. Serve with the salad.