Season brisket generously on both sides with salt and pepper.Whisk all sauce ingredients together in a bowl.(Optional) if your brisket will fit, heat a large cast iron skillet or dutch oven over medium-high heat. Cook for 3 to 5 minutes per side until browned.Place brisket in slow cooker. Cover with sauce. Cook on low heat for 10 hours, flipping once half way through (flip it very carefully using two large, flat spatulas rather than tongs so it doesn’t fall apart).Transfer the brisket and sauce to a baking dish, or you can also put the bowl of your slow cooker directly in the fridge if it will fit (let it cool slightly first). Cover and refrigerate for at least 3 hours or ideally overnight.Preheat oven to 300 degrees F.Remove brisket from fridge and spoon off any excess fat that has risen to the surface of the cooled brisket and surrounding liquid.Remove brisket from sauce and place on a large cutting board. Trim off any large fatty areas from the edges of the meat; slice into 3/8-inch thick slices and arrange in a baking pan.Transfer leftover liquid to a saucepan set over medium heat; bring to a simmer. Continue to cook until sauce has reduced by about a third, about 20 minutes.If you want a thicker sauce, prepare a roux by melting butter in a small skillet set over medium heat, then whisk in flour until smooth and lightly golden brown, about 5 minutes. Add roux to sauce, a spoonful at a time, returning to a simmer between each addition, until sauce is thickened and coats the back of the spoon.Pour sauce over top of sliced brisket in baking dish. Bake in preheated oven until heated through. Serve immediately.