To a small dish, combine two tablespoons of Japanese soy sauce, one tablespoon of sesame oil, one teaspoon of mirin, one teaspoon of katsuobushi bonito flakes, and one teaspoon of black and white sesame seeds, stir to combine, and set aside.
To a large bowl of warm water, add the brick of fresh udon noodles and leave to soak to loosen the strands of noodles so that they can easily be untangled. If vacuum-packed, the noodles should easily separate in 2-3 minutes. If frozen, it will take longer for the noodles to thaw so that they can easily be pulled apart. Once ready, drain the noodles and set them aside.
In a flat-bottomed wok over medium-high, heat the cooking oil until shimmering, add the onions and fry a few minutes until soft and fragrant, add the bacon and fry until cooked, then add the mushrooms, turning to ensure they’re covered in the bacon-infused oil. Let everything sit a little so the onions and mushrooms brown and bacon begins to crisp up, then turn the mushrooms over to brown before giving it all a final stir-fry and remove the wok from the heat.
Use a slotted spoon to scoop the crispy bacon, onion and mushrooms out and into a small dish so that the bacon oil stays in the wok.
Transfer the udon noodles to the wok and over medium-high heat, stir-fry the noodles so that they’re coated in oil, add the sauce, the scallions/spring onions, and two-thirds of the bacon, onion and mushrooms, and continue to stir-fry until combined and the noodles are hot. Don’t over-fry or the noodles will soften too much and stick to the wok.
Distribute the noodles between bowls, top with the remaining crispy bacon, onion and mushrooms, and press a tablespoon into the centre of the bowl to create a shallow well.
Carefully separate the egg yolk from the white and gently place the raw yolk in the centre of the bowl – or fry the eggs to your liking and slide the eggs into the well – and serve immediately with small dishes of sliced scallions/spring onions, bonito flakes and sesame seeds.