Put on pot of boiling water for pasta. Cook the fettuccine to al dente.
While the pasta is cooking, whisk together orange juice, peanut butter, soy sauce, vinegar, sugar, chili sauce and lime if you are using it.
Heat oil in skillet over med-high, once hot add the onion and garlic. Stir fry, until onion is cooked, but still has a little firmness, not mushy, then add the noodles and stir fry for 2 minutes,
Pour the sauce over noodles, and stir to coat, continue folding and mixing the pasta over heat for about a minute.
Serve with a splash of soy, Ponzu sauce, or wedges of lime.