About 30 minutes before the meat is done, place the bacon in a saucepan and cover with cold water. Bring to a boil and simmer for 5 minutes. Drain well. Placebacon in a frying pan over low heat and cook until it starts to brown. Drain on absorbent paper. Keep warm.
Cook the mushrooms in a frying pan over medium heat with a little water until tender. Salt and pepper to taste. Keep warm.
Place the pearl onions in a small saucepan with the butter and sugar over high heat. Cover the onions and bring to a boil. When the onions are tender and the water has evaporated, caramelize the onions a small amount. Add the vinegar and cook until it has evaporated fully. Keep warm.
When the meat is cooked, separate it from the sauce and vegetables. Purée the sauce and vegetables with a hand blender. Return the meat to the sauce and keep warm.
Divide the meat and sauce on the serving plates. Sprinkle the cooked bacon and mushrooms over the meat. Serve the onions on the side.