4 tablespoons olive oil, divided
1/4 cup finely chopped red onion
2 tablespoons red wine vinegar
1 1/2 teaspoons Dijon mustard
3/4 teaspoon fine salt, divided
1/4 teaspoon freshly ground black pepper
1 cup Israeli (pearl) couscous
2 cups water
4 ounces green beans, trimmed and cut into 1/2-inch pieces
4 ounces yellow wax beans, trimmed and cut into 1/2-inch pieces
1 (15-ounce) can kidney beans, drained and rinsed
2 tablespoons coarsely chopped fresh dill leaves
2 tablespoons coarsely chopped fresh Italian parsley leaves
Whisk 3 tablespoons of the oil, onion, vinegar, Dijon, 1/4 teaspoon of the salt, and pepper in a large bowl until combined; set aside while you cook the couscous.
Heat the remaining 1 tablespoon oil in a large saucepan over medium heat until shimmering. Add the couscous and cook, stirring occasionally, until toasted and light golden-brown, about 3 minutes. Add the water and remaining 1/2 teaspoon salt, stir to combine, and bring to a boil.
Reduce the heat to medium-low and simmer uncovered for 7 minutes. Add the green and yellow beans and stir to combine. Cover the pot and cook until the couscous and green beans are tender, about 3 minutes more. Drain the mixture through a fine-mesh strainer to remove any excess cooking liquid.
Transfer the hot couscous mixture to the bowl of dressing, add the kidney beans, and stir to combine. Let sit 20 minutes, stirring occasionally, to cool and let the flavors combine. Stir in the dill and parsley. Serve warm or at room temperature.