First of all, peel the onion, cut it into thin slices and sauté it in a large saucepan with oil and a pinch of salt.
Salt the meat, wrap it with the bacon, add some rosemary and tie it with string (here is the guide to do it correctly), then brown it in another non-stick pan with a little oil, turning it from all sides to seal them.
Add the onions and deglaze with the wine.
Once the alcohol has evaporated, add the milk and sage, cover with a lid and cook over medium-low heat for about 70 minutes (check every so often that it does not dry out too much).
After the cooking time, remove the meat and let it rest on the side wrapped in aluminum foil.
In the meantime, add the butter to the cooking juices and blend to obtain a homogeneous sauce: if needed, reduce it a little over medium heat.
Finally, take the meat back, remove the string and cut it into slices.