2 tablespoon packed dark brown sugar
2 tablespoon smoked paprika
2 teaspoon ground cumin
Kosher salt and freshly ground black pepper
1 5-lb. brisket
6 tablespoons tomato sauce
2 tablespoons apple cider vinegar
2 tablespoons honey
2 tablespoons molasses, preferably unsulfured
2 canned chipotles in adobo sauce, stemmed, seeded, and minced (about 2 Tbs.)
1 medium sweet onion, halved and sliced into thin half-moons
Mix the sugar, paprika, cumin, 1-1/2 teaspoon salt, and 1/2 teaspoon pepper in a small bowl. Rub onto both sides of the brisket.
Whisk the tomato sauce, vinegar, honey, molasses, and chipotles until smooth. Spread over both sides of the brisket.
Set the brisket in a 5- or 6-quart slow cooker. (The whole brisket should fit in an oval slow cooker; for a round model, cut the brisket in half and overlap the thinner sections.) Scatter the onion on top of the meat. Cover and cook until fork-tender, 8 to 9 hours on low or 5 to 6 hours on high. Uncover and transfer the brisket and onion to a cutting board; let rest while you finish the sauce.
Pour the sauce into a fat separator and set aside for a few minutes. Pour the defatted sauce into a 3-quart saucepan and bring to a simmer over medium-high heat, stirring occasionally. Cook until reduced by half, about 8 minutes.
Cut the brisket against the grain into 1/2-inch-thick slices and sprinkle with salt. Serve with the onion and some of the sauce