This isn’t “minimal-prep”, but you can make a batch up and store it in the fridge for future reference. And it smells so good while simmering….
(translated from a Japanese TV cooking course)
1 cup soy sauce (Japanese, not Chinese)
2/3 cup sugar
1/2 cup sake or sherry
10 cm long onion, or half a round onion, roughly chopped
1 knob ginger, thinly sliced
15 g cinnamon sticks
1 or 2 stars of star anise
15 g dried orange peel
Put all ingredients in pan, bring to boil, and simmer over low heat for approximately one hour, until liquid has reduced to about 2/3. Strain, cool, and store in fridge for up to one month. Use in same quantities as Oyster Sauce.