Soak 4 wooden skewers in water, 20 minutes. Meanwhile, whisk 1/4 cup olive oil, the vinegar, rosemary, mustard, 1/2 teaspoon salt, and pepper to taste in a large bowl. Reserve 2 tablespoons of the marinade, then add the steak to the bowl with the remaining marinade and toss; set aside at room temperature, 20 minutes.
Cook the couscous as the label directs; toss with the remaining 1 tablespoon olive oil, the parsley, and salt and pepper to taste.
While the couscous is cooking, preheat a large grill pan over high heat. Thread the steak onto the skewers. Toss the onion rounds and tomatoes in a bowl with the reserved 2 tablespoons marinade.
Brush the grill pan with olive oil and add the kebabs and onion rounds. Cook the onion, turning once, until charred and tender, 4 to 6 minutes. Cook the kebabs, turning a few times, until charred all over, 8 to 10 minutes for medium rare. Transfer the kebabs and onion to a plate and season with salt. Add the tomatoes to the pan, cut-side down, and cook, turning once, until charred and softened, about 4 minutes. Serve the kebabs with the couscous, tomatoes and onion.