How to Make It
In a blender, combine the onion, garlic, 1/2 cup of olive oil and 1 tablespoon of water and season with salt and pepper. Puree until smooth, then pour into a large resealable plastic bag. Add the steak, seal the bag and rub the steak with the marinade. Refrigerate for at least 2 hours or overnight.
Fill a medium bowl with ice water. In a medium saucepan of salted boiling water, cook the beans until crisp-tender, 2 to 3 minutes. Drain, then transfer the beans to the ice bath to cool. Drain well and pat dry. Halve the beans lengthwise and transfer to a large bowl. Add the remaining ingredients.
In a small bowl, whisk all of the ingredients.
Light a grill and oil the grate. Remove the steak from the marinade and season with salt and pepper. Grill over moderately high heat until lightly charred outside and medium-rare within, about 3 minutes per side. Transfer the steak to a cutting board and let rest for 5 minutes. Thinly slice the steak across the grain.
Add two-thirds of the vinaigrette to the bean salad, season with salt and pepper and toss. Serve the steak with the salad, passing the extra vinaigrette at the table.