Fill a large pot with lightly salted water and bring to a rolling boil. Stir in egg noodles and return to a boil. Cook noodles uncovered, stirring occasionally, until tender but still slightly firm, about 5 minutes. Drain well.
Melt butter in a large skillet or wok over medium heat; cook and stir onion until browned, about 8 minutes.
Cook and stir cabbage into onions until cabbage has wilted, another 5 to 8 minutes.
Gently stir cooked noodles into cabbage mixture; season with salt and black pepper to taste.
