Put 3 liters of water and 1 1/2 teaspoons salt in a large pot. Bring the salted water to a boil. Add udon noodles and cook for 9 minutes (or cook to packet instructions). Use a pair of tongs to remove the noodles and transfer to a colander. Give cooked udon noodles a quick rise and set aside.
Get a large bowl, fill it with water, then add ice.
Bring the salt boiling water in the pot down to a simmer. Add bok choy stems, simmer for 1 minute then add leaves and cook for another 30 seconds.
Use tongs or a large slotted spoon to transfer bok choy to ice bath. Leave for 20–30 seconds, then drain (do not leave the bok choy in the ice water).
Put stock, ginger, garlic and star anise in a pot. Bring stock to a boil over the stove, then turn heat down low and simmer stock uncovered for 10–15 minutes.
Once you have simmered the stock, use a slotted spoon to remove the ginger, garlic and star anise. Then use a regular spoon to skim the stock. Season stock to taste with soy sauce. Make sure you don’t add too much and taste as you go.
Season steak on both sides generously with salt and pepper.
Preheat a pan over high heat for at least 1 minute. Turn the kitchen fans on and/or open the windows.
When the pan is nice and hot, add oil. Add steak and sear for about 4 minutes on each side for medium rare (this is for a steak that is about 1 inch thick).
Transfer the steak to a plate to rest for a couple of minutes, then transfer to a board. Use a sharp knife to slice thin strips out of the steak.
Put some noodles in a serving bowl, then add some bok choy. Neatly place a few strips of steak on top. Garnish with spring onions and sesame seeds. Ladle in some of the stock and, if you’ve made them, serve with a ramen egg.