While onions are cooking, thinly slice or shred your cabbage. Aim for uniform slices/shreds for even cooking. Add cabbage to pan with onions and butter.
Cook the cabbage and onions 15-30 minutes, flipping and mixing regularly, until tender and showing a bit of browning.
Meanwhile, cook noodles according to package directions. Aim for “al dente”, not well done, so they hold up better in the finished haluski. Drain well.
Add the noodles to the cabbage mix (or the cabbage mix to the noodles, whichever way it fits better) and mix well. Add the sour cream and stir until distributed throughout the dish. Season with salt and pepper and serve immediately. Serves 6-10.