Preheat oven to 350 degrees Fahrenheit.
Mix together the veggies, tomato paste, and miso. Spread evenly out onto a rimmed baking sheet.
Roast veggies for 40 minutes, until they are soft and beginning to get a little blackened.
Toss the roasted vegetables in with the water, shiitake mushrooms, and kombu sheet. Bring to a boil and then reduce heat to low. Simmer for 30 minutes. Strain the vegetables and mushrooms out.
Heat the butter and olive oil in a large skillet over medium-high heat. Once the butter is melted and the olive oil is hot, add the onion and mushrooms. Cook for 6 to 8 minutes until well browned.
Add the flour and stir, cooking for 3 to 4 minutes until the flour is golden brown.
Slowly whisk in the vegetable stock until a smooth sauce forms. Turn heat down to low and simmer until the gravy reaches the desired consistency. Season with soy sauce, salt, and pepper. Strain out the solids through a fine-mesh sieve.
As you add the broth, don’t worry if the gravy clumps really badly. Just keep whisking and it will smooth out.
Don't strain out the solids if you like a chunky gravy.
The gravy recipe is based on Melissa Clark's Vegan Mushroom Make-Ahead Gravy of NYT's Cooking.