Slow-Cooker Brisket and Onions

1 Tbsp vegetable oil
3 large onions, halved and sliced ½" thick
1 Tbsp light brown sugar
Salt
1 Tbsp tomato paste
2 Tbsp all-purpose flour
3 garlic cloves, minced
1-3/4 cups low-sodium chicken broth
2 Tbsp plus 1 tsp red wine vinegar
1 Tbsp paprika
2 tsp onion powder
1 tsp garlic powder
1/8 tsp cayenne pepper
1 (5lb) flat-cut beef brisket, trimmed of excess fat
3 sprigs fresh thyme
3 bay leaves

Heat oil in large skillet over medium-high heat until shimmering. Cook onions, brown sugar, and ¼ tsp salt until onions are golden, 10-12 minutes. Stir in tomato paste and flour and cook until darkened, about 2 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in broth and cook until sauce thickens, about 4 minutes. Off heat, stir in 2 Tbsp vinegar and transfer mixture to bowl.

Combine 1 tsp salt, paprika, onion powder, garlic powder and cayenne in a bowl. Using fork, prick brisket all over. Rub spice mixture over brisket and wrap tightly in plastic wrap. Refrigerate brisket and bowl of onion mixture overnight.

The next morning, add half of the onion mixture to slow-cooker insert or roasting pan. Add thyme and bay leaves and place brisket, fat side up, on top. Spread remaining onion mixture over brisket. Set slow cooker to low, cover and cook until brisket is fork tender (9-10 hrs or cook on high for 5-6 hrs or use your oven at 325 degrees, cover the roasting pan with a lid and check every few hours). Turn slow cooker off and let brisket rest for 30 minutes. Refrigerate brisket then remove congealed fat from the top. Cut visible fat off the top of the brisket. Slice.

To Freeze: Freeze meat in sauce in freezer safe container.

To Reheat: Defrost. Reheat meat in a 350 degree oven for 1 hour. Add potatoes, carrots, celery, etc if desired.