Bring 1/2 cup plus 1 Tbs. water and a heaping 1/4 tsp. salt to a boil in a small saucepan. Add the couscous, give it a stir, cover tightly, and remove from the heat.
Put a heavy-based skillet that’s a bit bigger than the steak over medium-high heat, add the oil, and heat until very hot. Season the steak generously with salt and pepper and put it in the center of the pan. Add the beans, shaking the pan so they settle in the spaces around the meat. Sear the meat for about 3 minutes on one side, turn it, lower the heat to medium, and continue to cook about another 3 minutes for medium rare, 5 minutes for medium. Don’t move the steak around as it cooks, but shake the pan occasionally to brown the beans on all sides. Transfer the steak to a cutting board and tent it with foil. Leave the beans in the skillet.
Increase the heat to high, add the garlic, tomatoes, and about 1/4 cup water, and cook until the tomatoes give off some juice and start to collapse, 1 to 2 minutes. Scrape the pan to dissolve any of the browned bits. Season with salt and pepper and remove from the heat.
Uncover the couscous, fluff it by scraping it with a fork, and then fold in the beans and tomatoes. Taste for seasoning. Put a pile of couscous on each plate; cut the steak into thin slices and arrange them over the couscous. Pour any accumulated meat juices over the couscous and serve with the lemon, if using, to squeeze over all.