Put bacon and onion in a large covered skillet over medium-high heat; cook and stir until onions are translucent and bacon is still soft, about 5 minutes.
Cut each package of udon into quarters; add to the skillet. Cook and stir until noodles absorb bacon grease and fall away from each other, about 5 minutes.
Stir broccoli and napa cabbage into the skillet; sprinkle sesame seeds and black pepper on top. Cover the skillet and reduce heat to medium; cook, scraping and folding with a wooden spatula a few times, until tender, 5 to 7 minutes.