Preheat the oven to 325°F.
On a cutting board, mash the garlic and Â½ teaspoon of the salt together with the flat-side of a knife into a paste form. Add the rosemary and continue to mash until incorporated. Put the garlic-rosemary paste in a small bowl and add 2 tablespoons of olive oil; stir to combine.
Season both sides of the brisket with a fair amount of kosher salt and ground black pepper. Dust brisket generously with flour. Place a large roasting pan or Dutch oven over medium-high flame and coat with the remaining olive oil. Put the brisket in the roasting pan and sear to form a nice brown crust on both sides. Lay the vegetables all around the brisket and pour the rosemary paste over the whole thing. Add the wine and tomatoes; toss in the parsley, thyme and bay leaves. Cover the pan tightly with aluminum foil and transfer to the oven. Bake for about 3 to 4 hours, basting every 30 minutes with the pan juices, until the beef is fork tender.
Remove the brisket to a cutting board and let it rest for 15 minutes. Strain the stock and spoon out some of the excess fat.
Meanwhile, prepare the mushrooms. Heat up a large sauté pan and add a 2 count (approximately 2 tbsp) of olive oil. Add the chopped garlic, the mushrooms and thyme. Cook until a nice light brown color develops then add brisket stock and continue to simmer and reduce – mixture should be thick enough to coat the back of a wooden spoon. When thickened, finish with the horseradish, sour cream and season to taste with salt and freshly ground black pepper.
To serve, slice the brisket across the grain (the muscle lines) at a slight diagonal and top with the creamy wild mushrooms.