Umami is the Japanese name for savoriness, an essential characteristic of foods, like sweetness or saltiness. We sought out vegetables high in glutamate, an amino acid that plays an important part in umami, and gave them a quick char before simmering them down to a rich, flavorful stock used for gravy even a meat eater will enjoy.
Position an oven rack in the highest position, and preheat to 450ºF. 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.