Rub:
1 tablespoon plus 1 teaspoon kosher salt
1 teaspoon freshly ground black pepper
1/4 teaspoon smoked paprika
1/8 teaspoon ground cinnamon
Brisket:
1 4-pound trimmed flat-cut brisket with about 1/3' top layer of fat
2 tablespoons grapeseed oil, divided
3/4 cup chopped onion
3 garlic cloves, smashed
4 cups beef broth
3/4 cup bourbon
1/4 cup (packed) light brown sugar
1/4 cup soy sauce
6 large sprigs thyme
3 celery stalks, chopped
2 plum tomatoes, cored, chopped
1 large carrot, chopped
1 tablespoon balsamic vinegar
Glaze:
1/2 cup Mrs. Miller's Homemade Peach Jam
2 teaspoons bourbon
Kosher salt and freshly ground black pepper
Rub
Mix all ingredients in a small bowl.
Brisket
Rub brisket all over with spice rub. Cover and chill for at least 2 hours or overnight. Let stand at room temperature for 1 hour before continuing.
Preheat oven to 325°. Heat 1 tablespoon oil in a large wide pot over high heat. Add brisket, fat side down. Cook undisturbed until well browned, 5–6 minutes. Turn brisket over and cook until browned, about 3 minutes. Using tongs, transfer to a plate. Reduce heat to medium and add remaining 1 tablespoon oil. Add onion and garlic. Cook, stirring occasionally, until onion is slightly golden, about 5 minutes. Add broth and all remaining ingredients. Bring liquid to a simmer. Return brisket to pot. Cover and transfer to oven.
Braise until brisket is very tender to the touch but still holds its shape, about 4 1/2 hours. Using a large spatula, transfer brisket, fat side up, to a large plate. Strain braising liquid into a large bowl. Return liquid to pot, bring to a simmer, and cook until reduced to 2 cups, about 15 minutes. Score fat side of brisket by cutting a cross-hatch pattern of 1/4"-deep slits spaced 1/2" apart. Return brisket, fat side up, to pot with reduced braising liquid.
Glaze
Transfer 1/4 cup braising liquid to a blender. Add jam and bourbon and purée until smooth. Season with salt and pepper.
Preheat broiler. Spread 3–4 tablespoons glaze on top of brisket with the back of a spoon. Broil brisket in pot until browned and glazed, watching carefully to prevent burning, 4–5 minutes.
Transfer brisket to a cutting board. Slice against the grain and transfer to a large platter. Ladle braising liquid over. Drizzle remaining glaze on top.