- Using a mandoline, thinly slice the onions, shallots, carrots, beets, cauliflower, squash and garlic and place them in a large bowl.
- Add the miso and tomato paste and toss to coat the vegetables.
- Spread the coated vegetables evenly over a large sheet tray. Roast them in a 250-degree oven until they develop a rich, dark color, 30 to 60 minutes.
- Remove roasted vegetables from the pan and place in a large stockpot. Deglaze the pan with ¼ cup water.
- Add the mushrooms, thyme, sage, bay leaves and peppercorns. Cover with the water.
- Bring to a boil; then lower heat and let simmer over medium heat until the liquid has reduced by 75 percent.
- Strain through a fine-mesh strainer. Thicken with cornstarch slurry or flour, depending on your preference.