Cook potatoes until firm-tender (bake, steam or microwave). When cool enough to handle, cut potatoes into 1-inch chunks. Cook green beans in vegetable stock and refresh in cold water. Cook pasta. Heat 1/2 c. stock in saute pan; add onions and cook covered until tender. Add garlic and red pepper flakes and cook 2 minutes. Add tomatoes and tomato liquid, potatoes, greens and green beans and cook until greens are wilted and potatoes are heated through. Season with salt and pepper. Toss cooked pasta with warm sauce.