Bring a large pot of water to a boil; add a big pinch of salt and the green beans. Cook for about 5 minutes, the beans should still be crisp, they will be cooked more in the oven. Drain them and set aside.
Melt the butter and olive oil in a large skillet over medium heat. Add the onions, season with salt and pepper, and cook until the onions caramelize, about 20 to 25 minutes. Add the thyme, rosemary and chives, and cook for another 5 minutes. Add the green beans and walnuts, and stir well. Season, to taste.