1. Cook spaghetti according to package directions, omitting salt and fat; drain and keep warm.
2. Cut white portion of onions into 5 cm pieces; reserve green tops for another use. Cut squash in half lengthwise, and cut halves into slices. Remove and discard seeds from bell pepper, and cut pepper into thin strips.
3. Cook vegetables in a sauce pan in boiling water to cover 3-minutes; drain vegetables and set aside.
4. Pour oil into large skillet; place over medium heat until hot. Add garlic and cook, stirring constantly, 1 minute. Add peanut butter, stirring until smooth. Stir in soy sauce and next 3 ingredients.
5. Add vegetables to skillet, and toss gently to coat; remove vegetables from skillet with a slotted spoon.
6. Add spaghetti to sauce in skillet, tossing to coat; transfer to a serving plate, and top with vegetables. Serve immediately.