Heat oil in a large heavy-bottomed saucepan on medium heat.
Add onion, garlic and ginger. Sauté and cook for a few minutes until onions are glazed.
Add mushrooms. Toss occasionally and fry for 20-25 minutes until onion and mushrooms are caramelized. Reduce heat to medium-low if you feel they are browning too quickly.
Add water, increase heat to high and bring to a boil. Once the broth comes to a boil, reduce heat to low and simmer uncovered for 20 minutes.
While the broth is simmering, mix cooking sake, mirin, soy sauce, miso paste and tahini in a small bowl until smooth.
After the broth has simmered for 20 minutes, remove from heat and strain the broth, discarding the mushroom-onion mix. Return the liquid to the saucepan on low heat. Add the sauce mix to the pan, mix well and cook for 5-8 minutes until heated through. Remove and make your ramen.
Broth can be stored in the fridge for up to 4 days.
Notes
Cooking Sake - It is Japanese rice wine. It has a small amount of sweetness but is predominantly sour, tart and umami. It can be substituted with rice wine vinegar.
Mirin - It is a rice wine with higher sugar content and lower alcohol content. The sweetness is used to balance salty and sour flavours in a dish. It can be substituted with sweet sherry or just brown sugar in the same quantity.
Miso Paste - I like to use a mix of white and red miso paste. The addition of red miso paste gives it more depth of flavour and heightened colour. But any miso paste works well in this recipe.
Tahini - Tahini adds a rich creamy nuttiness to the broth. If you can find Chinese sesame paste, make that your first choice. Chinese sesame paste is made with toasted sesame seeds, making the paste darker and richer in its nutty flavour. Tahini on the other hand is made with raw sesame seeds, so the colour and taste will be more subdued if you use tahini. Peanut butter is another good substitution if you don't have either. My broth is made with hulled tahini.