Heat the oil in a large, heavy-bottomed pot over medium-high heat, until shimmering. Add the mushrooms and cook, stirring, until soft and brown, about 5 minutes. Add the onions and cook stirring, until soft and golden, about 8 minutes. Add the celery, carrots, peppers and garlic and cook, stirring, until the vegetables begin to brown and become fragrant, about 5 minutes. Stir in the soy sauce, garlic and onion powder, 2 teaspoons salt and 1 teaspoon pepper.
Add the cabbage to the pot and stir until it begins to wilt, about 2 minutes. Pour in the crushed tomatoes, chicken broth and 1 cup water. Stir to combine. Bring to a boil then reduce to a simmer and cook until the cabbage is tender, 30 to 45 minutes. Season with salt and pepper.
To speed up vegetable prep you can use a food processor fitted with the grater attachment. Add any liquid accumulated from the shredded vegetables to the soup along with the broth.
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