*If this has not been done already, tie the meat around its "waist" with cooking twine so it doesn't fall apart during cooking.
Heat oven to 350° F. Combine the porcini and broth in a small saucepan and bring to a boil. Remove from heat and let stand for 10 minutes. Remove the porcini with a slotted spoon, coarsely chop, and set aside. Strain the broth through a fine sieve and set aside.
Rub the meat with the salt and pepper. Heat 2 tablespoons of the oil in a roasting pan or large Dutch oven over medium-high heat. Cook the meat until well browned, about 2 to 3 minutes per side. Transfer the meat to a plate and set aside.
Reduce heat to medium, add the remaining oil, onions, cremini mushrooms, garlic, and rosemary to the pan and cook, stirring frequently, until the mushrooms and onions are tender, about 5 to 7 minutes. Return the meat to the pot and add the porcini, strained broth, vinegar, and sugar. Over high heat, bring to a boil. Cover (with aluminum foil if using a roasting pan). Transfer to the oven and cook until the meat is very tender, about 2 1/2 to 3 hours.
Transfer the meat to a cutting board and cover with foil to keep warm. With a slotted spoon, remove the vegetables and reserve. Place the pan over high heat and boil the cooking liquid until reduced by half, about 15 minutes. Add the parsley, stir, and season to taste with salt and freshly ground pepper.
To serve, cut the pot roast across the grain into thin slices. Place them on the platter with the vegetables and spoon on some of the sauce.