3 cups water
1¼ tsp. kosher salt, divided, plus more to taste
1 cup polenta, such as Bobs Red Mill
1½ Tbsp. unsalted butter or vegan spread
½ lb. small Brussels sprouts, trimmed
4 Tbsp. canola oil, divided
1 lb. whole button mushrooms, quartered
2 shallots, thinly sliced
2 tsp. minced garlic
1 Tbsp. flour
2 cups dry Marsala wine
1 Tbsp. dry sherry
1 tsp. fresh thyme
1∕8 tsp. freshly ground black pepper, plus more to taste
¼ cup chopped Italian parsley
In a large saucepan over high heat, bring the water and ½ teaspoon salt to a boil. Slowly add the polenta and stir constantly until it returns to a boil. Reduce the heat to medium-low and simmer, stirring frequently, until the polenta is creamy, 20 to 25 minutes. Stir in the butter, set aside and keep warm.
Meanwhile, cut the Brussels sprouts lengthwise into ¼-inch thick slices.
In a large cast-iron or heavy skillet over medium-high heat, warm 2 tablespoons oil. Add the Brussels sprouts and 1∕8 teaspoon salt. Saute, stirring occasionally, until the sprouts are bright green, tender and some leaves are charred, 5 to 8 minutes. Remove the sprouts, cover and set aside. Wipe out the skillet with a paper towel.
Heat 1 tablespoon oil over medium-high heat and add the mushrooms in a single layer with ⅛ teaspoon salt. Sear undisturbed 3 minutes, until the mushrooms are browned on one side. Flip and sear another 2 minutes, until browned and soft. Remove the mushrooms, cover and set aside. Wipe out the skillet and return it to the stovetop.
Warm the remaining 1 tablespoon oil over medium-high heat. Saute the shallots until soft and translucent, about 3 minutes. Add the garlic and saute 30 seconds, until fragrant. Sprinkle the shallots and garlic with the flour and saute 2 minutes. Add the Marsala, the sherry, the thyme, the remaining ½ teaspoon salt and the pepper, scraping up any browned bits from the bottom of the pan. Reduce the heat to medium and simmer about 7 minutes, until the sauce is reduced by half. Return the mushrooms to the skillet and heat through. Taste and adjust the salt and pepper.
Divide the polenta between the plates and add the mushroom mixture, then top with the Brussels sprouts. Garnish with the parsley before serving.