Bring a medium pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain. Place sun-dried tomatoes into a small pot, and add water to cover. Bring to a boil over medium-high heat, and cook for about 5 minutes, or until tender. Drain, chop, and set aside.
Heat olive oil in a large skillet over medium-low heat. Fry onion and garlic in the oil until onions are transparent, about 5 minutes. Add the mushrooms and sun-dried tomatoes; cook over low heat for about 5 minutes. Throw in the spaghettini, and toss to coat. Mix in the green olives. Serve hot with bread crumbs and lots of black pepper on top.