Simple Udon Noodle Soup (Kake Udon)

Name: Simple Udon Noodle Soup (Kake Udon)
Prep time: 35 min
Cook time: 25 min
Total time: 1 h
Cuisine: Japanese
Ingredients: 10 g dried sardines (niboshi) (substitute for dried shiitake mushrooms for plant-based diets); 10 g dried kelp (kombu); 15 g bonito flakes (katsuobushi) ((katsuobushi) omit for plant-based diets); 1000 ml water; 5 tbsp Japanese soy sauce (koikuchi shoyu); 2 tbsp mirin; ½ tsp salt; 5 portions udon noodles; 5 tbsp finely chopped green onions; toppings of your choice (see in post)