Preheat the oven to 300f.
The next day, strain and reserve the liquid, vegetables, and brisket all in separate bowls.
Dry the brisket very well with paper towels. Season the brisket with salt and pepper on all sides then dredge in flour and shake off the excess.
Heat an oven-safe roasting pan to medium heat. Add the olive oil and the pancetta and cook until crisp (7-10 minutes) then remove to a plate with a slotted spoon and set aside.
Add the brisket to the pan and brown it on all sides. Once the brisket is nicely browned, remove it to a plate and set aside.
Add the reserved vegetables to the pan and cook until softened and lightly colored (about 10 minutes).
Add the liquid back to the pan and bring to a boil. Scrape the bottom with a wooden spoon to remove all of the flavor bits then turn the heat off. Add the brisket (fat side up), pancetta, and sachet back to the pot. Cook in the oven covered for 4 hours covered very tightly with a double layer of foil. After 4 hours check for doneness. The brisket must be tender (a knife or fork will push through the meat with very little resistance). If the brisket is tough, return to the oven for another 45-60 minutes and check again. For more on this see notes below.
When finished remove the brisket from the oven and let sit for 30 more minutes without removing the foil cover. After 30 minutes remove and discard the sachet. Place the pan onto a burner and turn the heat to medium-high. Boil the sauce until thickened to your liking. Traditionally the sauce should coat the back of a wooden spoon, but not be overly thick.
Add the marsala and cook for 1 more minute. Whisk the butter and stir to incorporate. You can blend the sauce with an immersion or regular blender to further thicken it if you like or leave it as is for a more rustic appearance. Taste test the sauce and season with salt and pepper if required.
Cut the brisket against the grain and spoon the sauce all over the meat in a serving platter. Serve with polenta or mashed potatoes. Enjoy!