Heat vegetable oil in a large skillet over medium-high heat.

Sauté sliced onions with brown sugar and salt until golden brown (10-12 minutes).

Clear a space in the pan, add tomato paste and flour, and cook until darkened (2 minutes).

Add minced garlic and cook until fragrant (30 seconds).

Stir in chicken broth and cook until sauce thickens (4 minutes).

Remove from heat and stir in 2 tablespoons of red wine vinegar. Refrigerate the onion mixture overnight.

Whisk together salt, paprika, onion powder, garlic powder, and cayenne pepper in a small bowl.

Prick the brisket evenly with a fork on both sides.

Rub the spice mixture over the brisket and wrap tightly in plastic. Refrigerate the brisket overnight along with the onion mixture.

The next morning, add half of the onion mixture to the slow cooker.

Add thyme and bay leaves, and place the brisket on top, fat side up.

Spread the remaining onion mixture over the brisket.

Cover the slow cooker and cook on low for 9-10 hours (or on high for 5-6 hours) until the brisket is fork-tender.

Turn off the cooker and allow the brisket to rest for 30 minutes.

Remove the brisket to a cutting board.

Cut across the grain into 1-2 inch slices and transfer to a serving platter. Tent with foil.

Pour the sauce into a large skillet, discard the herbs, and simmer over high heat until slightly thickened (8-10 minutes).

Skim off as much fat as possible, add the remaining vinegar, then pour half of the sauce over the brisket.

Serve with the remaining sauce on the side.