Bring a large pot of salted water to a boil.
In large saucepan, heat olive oil over medium heat. Add onion and garlic; cook and stir until tender, about 5 minutes.
When water comes to a boil, add the spaghetti and cook according to package directions until al dente.
Meanwhile, add broth, peanut butter, ginger, brown sugar, soy sauce and cayenne pepper to onions. Bring to a simmer and cook, stirring for 3 minutes.
When pasta is done, drain, reserving 1/2 cup pasta cooking water. Add pasta to saucepan; cook and stir until pasta is coated, about 1 to 2 minutes. Add reserved cooking water as needed to make a smooth sauce. Sprinkle with peanuts and serve immediately.