Pour 8 cups vegetable stock into a medium pot and drop in 8 shiitake mushrooms. Bring stock to a boil over high heat, stir occasionally until boiling, then remove from heat and let mushrooms steep for 30 minutes. Pour stock and mushrooms into a large measuring cup or bowl, and wipe out the pot with a paper towel.
Add ½ cup butter to the empty pot and melt over medium heat. Add 1 shallot, and cook for a minute or so until softened but not brown, whisking a few times. Turn heat up to medium-high and add the ½ cup plus 3 Tablespoons flour and cook, whisking, until it turns very lightly golden, about two minutes. It will be quite lumpy, don't worry, this is fine.
Pour in the stock with the mushrooms, whisking while adding to the flour mixture. The lumps will even out as you pour and whisk. Add 2 Tablespoons soy sauce, and about ¼ teaspoon fresh ground pepper (or to your liking- you'll taste again later to add additional if desired).
Bring mixture to a boil, then reduce heat and simmer, uncovered, until gravy is reduced by at least a third, and to your liking, thickness wise (about 20 minutes, depending on how hard of a simmer is going on). Taste for seasoning and add 1 teaspoon salt (more or less, as desired) and additional pepper if needed.
Remove mushrooms before serving. If you want the gravy to be silky smooth, pour it into a large bowl through a sieve, pushing with a spatula. This will remove the minced shallots and any lumps if there are any.