Note: Despite my enthusiasm for this mixture back in 2004, I no longer make this. I found it wasn't as versatile and usable-all-the-time as I thought. Instead, I make kaeshi  periodically, and make as much dashi as I need for a few days and put that in the refrigerator instead. The recipe remains here anyway - you may find it more useful than I did. It just shows that we don't stay the same as cooks!
I’ve got an amazing bottle in my refrigerator now. It’s filled with a mixture that forms the base for just about any sort of Japanese food. It takes all the drudgery out of making a clear soup, or a Japanese style stew, or the dipping sauce for noodles. I can’t live without it anymore.
No this is not some amazing new product. It’s just a simple mixture that can be made in about 20 minutes, but it really is a great “essence of Japanese flavor”. It was in a Japanese magazine my mother brought for me last month. If you like Japanese food, you might want to make a bottle of this too. It will keep for up to three months in a closed container in the refrigerator.
It is simplicity itself to make, but some of the ingredients may be unfamiliar to you. You can get them all at a Japanese food shop.
To make the essence, combine 1 1/2 cups of soy sauce, 1 cup of mirin, 1 1/2 cups of sake, about 3 pieces of konbu, and a huge handful of the bonito flakes in a pan. Bring to the boil then lower the heat and simmer it gently until the liquid is reduced to about 2/3rds. Let it cool, then strain through a fine sieve and store in a jar or bottle in the refrigerator.
That’s all there is to it! You can use this as a sauce for vegetables, or tofu, or meat…add a bit of sugar to make it a teriyaki sauce…add water or basic Japanese stock  to make a dipping sauce for cold noodles (soba or udon, etc)…add hot stock or water to make a soup for hot noodles…add some lemon juice or rice vinegar plus oil to make a Japanese style salad dressing….and on and on. It really is great stuff!
Example: to make chicken (or pork, or beef..) teriyaki, saute a piece of boneless chicken (either breast or thigh) until browned on both sides. Add the essence and optionally a little sugar, and cook on high until the essence has been reduced to a dark, rather sticky sauce. Delicious!