In a small saucepan, bring the porcini mushrooms and 1 1/2 cups stock to a boil, reduce the heat to low and simmer until the mushrooms are softened, about 10 minutes. Using a slotted spoon, transfer the porcinis to a work surface. Coarsely chop; reserve the liquid.
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; cook, stirring often, until browned, about 10 minutes. Add the tomatoes, onion, celery, rosemary, garlic, bay leaf, sausage and chopped porcinis. Pour in the reserved liquid, leaving behind any grit. Season with salt and pepper. 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. Reserve the brown butter.
In a medium 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 to 5 minutes. Stir in the cheese, nutmeg and brown butter; season.
Line shallow bowls with the polenta; make a well. Fill with ragu. Garnish with the sage.