Bring the chicken stock to a boil in a small saucepan. Remove from the heat and add the dried porcini.
Cover and let steep for 20 minutes. Drain the porcini, pressing down on them to extract as much liquid as possible. Reserve the liquid and discard the porcini.
Heat 1 tablespoon of the oil in a large enameled cast-iron casserole. Season half of the pork with salt and pepper and cook in a single layer over high heat, turning once, until browned, about 4 minutes.
Transfer the meat to a plate. Repeat with the remaining oil and pork. Add the onion, garlic, sage, 1 1/2 teaspoons of salt and 1/2 teaspoon of pepper to the casserole and cook over moderate heat, stirring often, until the onion softens, about 5 minutes.
Stir in the Portobello pieces and cook until their volume is reduced by two-thirds, about 10 minutes.
Add the wine, raise the heat to high and boil until reduced by half, about 3 minutes. Stir in the tomatoes and tomato paste.
Pour in the reserved mushroom liquid, stopping when you reach the grit at the bottom.
Bring to a boil, reduce the heat to moderately low and simmer for 20 minutes.
Add the pork and its juices to the casserole and simmer gently until the pork is just cooked through, about 5 minutes. Season with salt and pepper.
Remove from the heat, stir in the chopped parsley and serve.