This delicious salad is hearty enough to be a meal.
Preheat oven to 400 degrees. On a rimmed baking sheet, toss bread with 2 teaspoons oil and Parmesan, then season with salt and pepper. Bake until golden brown, 7 minutes.
Heat a grill pan or cast-iron skillet over medium-high. Season steak with salt and pepper. Drizzle onions with 1 teaspoon oil and season with salt and pepper. Grill steak and onions until steak is medium-rare and onions are tender, 5 to 7 minutes, flipping once. Let steak rest 5 minutes before thinly slicing against the grain.
Meanwhile, in a large bowl, whisk together mustard, vinegar, and 2 tablespoons oil. Add arugula and season with salt and pepper. Add onions, steak, and croutons and toss to combine. Serve immediately.