Preheat the oven to 350 degrees F.
Season the meat liberally with salt and pepper. Add a couple of tablespoons olive oil to a large Dutch oven, a couple of turns of the pan, and heat over medium-high heat. Add the meat and brown all over, 10 to 12 minutes. Remove the meat from the pan and drain off meat fat if any accumulates. Add another turn or 2 of the pan with extra-virgin olive oil. Add onions, garlic, bay and season with salt and pepper, cook to tender 8 to 10 minutes, add in a few pinches cloves and stir in the tomato paste 1 minute. Next stir in the wine and deglaze the pan, then add the stock and canned diced tomatoes and stir to combine. Settle the meat back into the pot and cover. Bake 1 1/2 hours, then uncover and bake 1 hour to reduce the liquids, then place the cover back on and cook until very tender, 1 1/2 hours more.
Remove the brisket to the carving board and cover with foil to keep warm.
Reduce the sauce a few minutes, simmering gently, uncovered to thicken a bit.
Heat 3 tablespoons extra-virgin olive oil over medium-high heat and add garlic and mushrooms, cooking until very tender, season with salt and pepper and stir in sage. Bring the stock-milk mixture to a boil in a saucepot and add the polenta. Whisk the polenta 2 to 3 minutes until it pulls away from the side of the pot. Add the butter, if using and fold in mushrooms.
Slice the brisket and serve with sauce over polenta with mushrooms. Cool half of the brisket and store for later in the week. Scatter a few bitter greens over the top for an optional garnish.