Stir the dark soy sauce, Oyster sauce, Hoisin sauce, and mirin together in a small bowl until smooth. The sauce should look dark and glossy so it coats the beef evenly when it hits the pan.

Heat the oil and sesame oil in a medium skillet over high heat, then add the onion and garlic and cook for 1 1/2 minutes. You want a lively sizzle and a fragrant aroma, with the onion just starting to turn golden at the edges.

Add the beef mince and break it up as it cooks until it changes from pink to light brown. Once the meat stops steaming and starts sizzling more sharply, it is beginning to brown instead of just releasing moisture.

Pour in the sauce and keep cooking for 2 to 3 minutes, stirring so the beef caramelizes in it. The skillet should smell rich and savory, and the beef should look darker with sticky browned bits rather than wet and loose.

Push the beef mixture to one side of the skillet, pour in the water, and lay the noodles in the liquid. The noodles should sit mostly in the water so they can soften quickly without washing the sauce off the beef.

Leave the noodles for 45 seconds, turn them, then leave them for 30 seconds more before loosening them apart. They should look pliable but not fully soft, which helps them finish cooking without going mushy.

Toss the noodles through the beef mixture until the strands are separated and coated. The noodle starch will start to thicken the liquid into a glossy glaze that clings to the noodles instead of running to the bottom of the pan.

Add the bean sprouts and toss for 1 minute, then serve immediately with green onion / shallots and sesame seeds. The sprouts should just soften while the sauce reduces enough to coat the noodles and the noodles should be fully cooked but still springy.