This warm and comforting dish is perfect for winter nights.
Place the dried mushrooms in a medium mixing bowl, and cover with boiling water. Let the mushrooms stand until softened, about 20 minutes. Lift the mushrooms from the water, and gently squeeze the liquid back into the bowl. Cut the larger mushrooms in half, and strain the liquid through a cheesecloth-lined strainer; set aside.
In a large high-sided skillet, combine the olive oil and 2 tablespoons butter, and place the skillet over medium-high heat. Add the white button mushrooms, and saute, stirring frequently, until the mushrooms are golden, 10 to 12 minutes. Add the onions, garlic, celery, savory, thyme, marjoram, and adjust the seasoning with salt and pepper to taste. Reduce the heat to medium low, and saute until the onions are translucent and softened, about 10 minutes.
Add the dried mushrooms, reserved mushroom liquid, tomatoes, Marsala, and 2 cups chicken stock, and bring to a boil. Reduce heat to medium, and simmer the ragu until the liquid has thickened slightly and is reduced by two-thirds, 35 to 40 minutes.
Meanwhile, in a medium saucepan, combine the remaining 5 cups chicken stock, 2 tablespoons butter, and 1 teaspoon salt, and bring to a boil. While whisking rapidly, add the polenta in a thin steady stream. Reduce the heat to low, and cook the polenta, stirring occasionally, 3 to 4 minutes. Divide the polenta among six plates, cover each serving with the mushroom ragu; serve immediately.
For lump-free polenta, hold a fistful of cornmeal over the boiling stock, and let the cornmeal trickle out through a small opening in your fist. With your other hand briskly whisk continuously, until all the grain is absorbed.