Preheat the oven to 375 degrees . In an ovenproof skillet, heat the olive oil over medium heat. Add the garlic and cook for 1 minute. Add the bread cubes and toss to coat; transfer to the oven and bake until golden, about 10 minutes.
Meanwhile, in a medium saucepan of boiling water, cook the beans until crisp-tender, about 2 minutes; drain and let cool. Transfer to a bowl. Add the spinach and toss.
Preheat an outdoor grill or grill pan to medium-high. Season the steak with salt and pepper. Grill for 6 minutes on each side for medium-rare; cool, then thinly slice.
Meanwhile, in a small bowl, mash the blue cheese and buttermilk until no large clumps remain. Whisk in the mayonnaise and vinegar; season with pepper. Divide the spinach, green beans, steak slices and croutons between 2 plates. Drizzle the blue cheese dressing on top.