Bring a large pot of water to a boil and cook the noodles according to the package directions. Drain and set them aside.
In a small bowl, stir together the peanut butter, soy sauce, and rice vinegar until smooth. Add a splash of water if the sauce feels too thick.
Heat the vegetable oil in a large skillet over medium-high heat. Add the garlic and onion and cook for 1 minute until fragrant.
Add the carrots and bell pepper to the skillet. Stir-fry for 3 minutes until they begin to soften.
Stir in the shredded cabbage and cook for another 2 minutes until the cabbage wilts slightly.
Add the drained noodles to the skillet along with the peanut sauce. Toss everything well so the noodles and vegetables are evenly coated.
Remove from heat and sprinkle with the sliced green onions before serving.