Pat brisket with paper towel and season with salt and pepper.
Heat Arbequina EVOO in an extra large skillet (or you can sear your brisket in a heavy bottom roasting pan) over medium-high heat. Once the olive oil is hot, carefully add the brisket. Sear the brisket until richly seared and dark brown appx. 2-3 minutes per side. Remove from the pan and set aside on a large plate or platter.
Add the onions, shallot, and garlic to the same pan and sauté for 1-2 minutes. Deglaze the pan with sherry wine (using your spoon to scrap up any browned bits) and then bring to a simmer.
Once simmering, add the Sherry Vinegar, broth, tomato paste, rosemary, and bay leaves. Stir to combine.
Return the brisket to the pan (and any pan juices). Nestle the brisket snuggly among the onions. Cover the pan and plan in the oven. Roast for 4-6 hours or until the brisket is fork tender (this process may take longer depending upon your oven and brisket cut).
Remove brisket from the oven and set aside to rest for 30 minutes before slicing and serving. Serve with onions on top and garnish with fresh rosemary.