In a large saucepan, combine the wine, vegetable broth, parsley sprigs, thyme, and bay leaf. Bring to a simmer and cook for about 25 minutes until reduced to roughly 3 cups.
Meanwhile, heat 2 tablespoons olive oil in a Dutch oven over medium-high heat. Season the king oyster mushroom medallions with salt and pepper, then sear them until golden brown, about 3 minutes per side. Transfer to a plate. (The key here is getting a proper sear on these mushrooms — they'll develop incredible umami that rivals any meat)
Add remaining oil to the pot and brown the cremini mushrooms until caramelized, about 5 minutes. Transfer to the plate with the king oyster mushrooms.
Melt 2 tablespoons vegan butter in the same pot over medium heat. Add pearl onions and cook until lightly browned, about 7 minutes.
Add garlic, tomato paste, and flour to the onions and cook for 1 minute until fragrant.
Remove the bay leaf from the reduced wine mixture, then pour it into the Dutch oven, scraping up any browned bits from the bottom. Add 0.25 teaspoon black pepper.
Return all the mushrooms to the pot and bring to a boil. Reduce heat, cover, and simmer for 25 minutes, stirring occasionally.
Transfer the mushrooms to a bowl and tent with foil. Increase heat and simmer the sauce until thickened, about 5 minutes.
Remove from heat and stir in remaining 2 tablespoons vegan butter and a splash of wine. Season with salt to taste, return mushrooms to the pot, and sprinkle with fresh parsley.
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