Position an oven rack in the highest position, and preheat to 450 degrees F. Toss the onions, celery, carrots and garlic with 1 teaspoon each oil and salt on a rimmed baking sheet. Roast until the vegetables are charred in some places, 25 to 30 minutes.
Bring the roasted vegetables, 1 cup water, mushrooms, thyme, sun-dried tomatoes, soy sauce, bay leaf and 1 teaspoon salt to a simmer in a medium saucepan over medium-high heat. Cook, stirring occasionally, until most of the liquid is gone, 8 to 10 minutes. Add 6 cups water and 1/2 teaspoon salt, and bring back to a simmer. Reduce the heat to medium-low, and gently simmer until reduced by about one third, about 45 minutes. Strain the stock through a strainer into a large liquid measuring cup; use the back of a ladle to squeeze all the liquid out of the solids (there should be about 4 cups of stock). If not making gravy right away, let the stock cool to room temperature, then refrigerate for up to 3 days or freeze for up to 1 month.
To make the gravy, warm the stock slightly in a medium saucepan if it has been refrigerated. Heat the remaining 4 tablespoons oil in another medium saucepan over medium heat. Add the sage leaves, and cook until fried and crispy, about 2 minutes; remove and set aside. Add the flour to the saucepan, and stir until smooth and lightly golden, about 2 minutes. Slowly pour in the warm stock while whisking constantly until smooth and thick, 8 to 10 minutes
Chop up the fried sage, stir it into the gravy along with the parsley and season to taste with pepper.
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