Pat the beef brisket completely dry with paper towels. In a small bowl, combine rosemary, thyme, oregano, salt, pepper, and smoked paprika. Rub the herb mixture generously over all sides of the brisket. Let rest at room temperature for 15 minutes.

Heat olive oil in a large skillet over medium-high heat. Sear the brisket fat-side down for 4-5 minutes until golden-brown. Flip and sear the other side for 4 minutes.

Arrange sliced onions across the bottom of your slow cooker. Scatter smashed garlic cloves and bay leaves over the onions. Add optional vegetables if using.

In a medium bowl, whisk together beef broth, balsamic vinegar, tomato paste, Worcestershire sauce, and brown sugar until smooth.

Place the seared brisket on top of the onion layer, fat-cap facing up. Pour the braising liquid around (not over) the meat.

Cover and cook on low for 8-10 hours or high for 5-6 hours, until a fork slides in easily and internal temperature reaches 195-205°F.

Transfer brisket to a cutting board and tent with foil. Rest for 15-20 minutes. Slice against the grain in 1/4-inch thick pieces.

Optional: Strain cooking liquid, skim fat, and simmer in a saucepan for 10-15 minutes until reduced by half. Drizzle over sliced brisket before serving.