Place the potatoes in a pot, cover with water and bring to a boil. Salt the water and cook until the potatoes are fork-tender, 12-15 minutes. When tender, drain the potatoes and return to the hot pot. Mash the potatoes with the milk, cream or stock, sage, salt and pepper, and the cheese. Add nutmeg to taste and stir in the egg.
While the potatoes come to a boil, place the porcinis in a small pot and cover with the stock. Place over medium heat, bring to a boil, then reduce the heat to low and simmer to reconstitute the mushrooms.
While the mushroom s are softening, heat the EVOO in a skillet over medium-high heat, place an oven rack in the center of the oven, and preheat the broiler. Add the pancetta to the hot oil and crisp for 2-3 minutes, then add the chicken and cook for 5-6 minutes. Add the shallots and carrots and cook for 5 minutes more. Sprinkle the veggies with the flour and cook for 1 minute, then add the wine.
Remove the porcinis from the stock and stir the stock into the chicken mixture. Chop the porcinis and stir into the mixture. Thicken the sauce for 2 minutes and scoop the chicken into a 9x14" casserole or individual casseroles. Top with the potatoes, brown under the broiler for a couple of minutes, and serve.