Heat oil in a large nonstick pan. Add onion, pepper, celery, carrots, and beans. Cook over medium heat until vegetables are tender yet crisp, about 5 minutes. Add tomatoes, peas, artichokes, garlic, and tarragon and cook 2 minutes longer. Add tomato juice and bring to a simmer. Add spinach and simmer 1 minute longer. Season with salt to taste. Serve over couscous or rice, if desired.