In a large pot of boiling salted water, cook spaghetti 3 minutes less than al dente. Add broccoli, bell peppers, and onion. Cook until pasta is al dente and vegetables are tender, 3 minutes more. Reserve 1/2 cup pasta water; drain pasta and vegetables.
Meanwhile, in a large bowl, whisk together peanut butter, sugar, vinegar, soy sauce, oil, garlic, and red-pepper flakes. Add hot pasta and vegetables; toss to coat, thinning sauce with