In a small saucepan, bring the porcini mushrooms and 1 1/2 cups stock to a boil. Reduce heat to low and simmer until the mushrooms are softened, about 10 minutes. Using a slotted spoon, transfer the porcinis to a cutting board and coarsely chop. Reserve the porcini-infused stock.
In a large skillet, heat the oil, two turns of the pan, over medium-high. Add the sausage and cook, stirring occasionally and breaking up with a spoon, until browned; transfer to a bowl. Add the cremini mushrooms and cook, stirring often, until browned, about 10 minutes. Add the tomatoes, onions, celery, rosemary, garlic, bay leaf, browned sausage and chopped porcinis. Pour in the reserved stock, leaving behind any grit that may have settled in the bottom of the pan. Season with salt and pepper, and simmer, stirring often, until the ragu thickens, about 20 minutes. Discard bay leaf.
In a small saucepan, melt the butter over medium heat. Add the sage; cook until sage is crispy and the butter browns, about 5 minutes. Using a slotted spoon, transfer the sage to a paper towel to drain and remove pan from heat; reserve.
In a medium-size pot, bring the remaining 2 1/2 cups stock and the milk to a low boil; gradually whisk in the polenta. Cook, stirring often, until thick, 3-5 minutes. Stir in the cheese, nutmeg and brown butter; season to taste.
Line shallow bowls with the polenta. Make a well in the center of the polenta and fill with ragu. Garnish with the crispy sage.