Bring a large pot of salted water to a boil for the pasta. While water is heating, peel and finely chop the onion. Core, seed and chop the red peppers. Wipe the mushrooms clean with a damp paper towel, remove the stems (you can reserve them for another use-such as making stock), and thinly slice the caps. Add the pasta to the boiling water and cook until tender but still firm to the bite, about 8 to 10 minutes. Meanwhile, heat oil in a large skillet over medium-high heat. Stir in onions and peppers, cover, and reduce heat. Simmer until the vegetables are tender, about 5 minutes. While this is cooking, trim the green beans and cut them into 1-inch pieces. Rinse, pat dry, and mince the sage. Add the chicken broth, mushrooms, green beans, and sage to skillet. Cover and simmer over low heat until all the vegetables are tender, about 5 minutes. Season with salt and pepper. Drain the pasta and portion it out. Ladle some of the vegetables and sauce over each portion, and crumble the goat cheese on top if you wish. Serve immediately.