Make a small incision to the meat (one that just fits a garlic clove) using a sharp knife. Stuff it with lots of pepper (about ⅓ of a teaspoon), kosher salt, and a whole garlic clove. Make about 7-8 incisions here and there, exactly the same way. Keep the remaining garlic cloves to use in the sauce. At this point, you can tie the meat. See my post on How To Tie A Roast.
Heat a good splash of olive oil in a medium-sized cooking pot, over high heat.
Sear the meat on all sides giving it a brown color. Remove from pot and set aside.
Reduce heat to medium-low and in the same pot cook the onions until completely tender and golden in color.
Add the remaining garlic cloves, bay leaves, and fresh rosemary. Pour in the wine. Lemon juice, and lemon zest.
Add the meat back in to the pot and wait until wine completely evaporates (it stops smelling of alcohol).
Cover the meat with hot water and season with salt. Keep in mind the salt you added inside the meat. Add the carrot (no need to cut it we just want to get its flavor).
Reduce heat to medium-low and simmer covered for approximately 3 hours. Prick the meat to check if it feels tender in the center.
Remove the meat from the pot and set it aside on a cutting board.
Remove the bay leaves, carrot, and rosemary from the sauce. Blend the sauce using an immersion blender. Weigh the leftover sauce. For every cup of sauce, you need one tablespoon of cornstarch to thicken it. Or for every 1 liter of sauce, you need 4 tablespoons of cornstarch.
After you have weighed the sauce and calculated how many tablespoons of cornstarch you need, add the sauce to the pot and add the cornstarch to a mixing bowl. Add as much cold water as needed in the bowl with the cornstarch in order to create a runny paste.
Heat the sauce over medium heat. Using a ladle pour spoonfuls of the hot sauce in the bowl with the cornstarch while you stir the cornstarch mixture with a hand whisk. Once you have added most of the sauce into the bowl with the cornstarch, transfer the mixture back to the pot.
Let the sauce get back to a simmer and start to thicken. Taste it for any additional seasoning and if you like it more sourish add the juice and zest of 1 more lemon. Turn the heat off and mix in the butter.
Cut the meat into thin slices and place in a large pan or dutch oven. Pour the hot sauce on top. Cover and let it stand for at least 30 minutes in the sauce before serving.