Wafu Roast Beef (Japanese Style Roast Beef with Homemade Sauce)

Name: Wafu Roast Beef (Japanese Style Roast Beef with Homemade Sauce)
Prep time: 30 min
Cook time: 20 min
Total time: 50 min
Cuisine: Japanese
Ingredients: 450 g beef roasting joint (I used top round); ½ tbsp cooking oil; 2 cloves garlic (roughly chopped); 3 slices ginger root; 1 Japanese leek (naganegi) (or regular leek, green part for simmering, white part finely diced for the sauce); 120 ml dashi stock; 4 tbsp Japanese soy sauce (koikuchi shoyu); 4 tbsp sake; 4 tbsp mirin; 4 tbsp grated daikon radish (daikon oroshi) (for best results, grate with a Japanese-style "oroshi" grater); finely chopped green onions (to garnish); wasabi (optional)