Place a 4-quart pot over medium-high heat. Swirl in just enough oil to coat the bottom of the pan. Add the mushrooms, season with salt, and sauté, stirring occasionally, until deep, golden brown, 7 to 8 minutes.
Add the onions, and use a wooden spoon to scrape up any browned bits of mushroom. Cook until the onions begin to slightly soften, about 2 minutes. Give the mushrooms and onions a taste, and add more salt if necessary.
Add the carrots and garlic, giving everything in the pot a good stir. Cook for 2 more minutes.
Use a wooden spoon again to scrape up any browned bits on the bottom of the pot. Pour in the water, and add the herbs, and soy sauce. Increase the heat to high, and bring to a boil. Reduce to heat to low, and simmer until the liquid has reduced by 1/3, 35 to 40 minutes.
Place a sieve, or fine meshed strainer, over a deep pot. Pour the stock through the sieve, to strain out the vegetables and herbs. The stock is now ready to use, or you can portion it into smaller containers for use later on. Store it in the fridge, covered, for up to one week, or the freezer for up to two months.