Heat a heavy based frying pan until smoking hot. Season the beef with salt & rub a little oil onto the surfaces. Sear the beef all over, this will take 4-5 minutes until the beef is rich & caramelised. Remove from pan & keep warm whilst you assemble the broth. Heat the sesame oil in a large pan over medium heat. Add the garlic and ginger, and cook for a few minutes until softened. Add the mushrooms, continue to cook until tender & nicely browned. Next add the soy sauce & stock. Bring to a boil and simmer for 5 minutes. Add the Udon noodles and cook for 3-4 minutes (or according to package directions). Stir in the greens & radishes then divide into warmed serving bowl. Slice the beef into thin strips & scatter over the broth.