1. Heat oil in large saucepan over medium-high heat. Add leeks, bell pepper, and red pepper flakes; sauté 10 minutes, or until vegetables begin to brown. Stir in beans, tomatos, vegetable broth, and garlic. Reduce heat to low, and simmer 5 minutes. Season with salt and pepper, if desired.
2. Meanwhile, cook spaghetti according to package directions. Drain.
3. Mix sauce and spaghetti, top with toasted walnuts before serving.
Modified from original recipe in Vegetarian Times