Heat the vegetable oil in a large skillet over medium-high heat. Season the brisket with salt and pepper, then brown on all sides, about 10 minutes, adding the garlic in the last 2 minutes. Transfer the meat and garlic to a 5-to-6-quart slow cooker. Pour the beer into the skillet and simmer 30 seconds, scraping up the browned bits from the pan; add the beer mixture to the slow cooker.
Nestle the celery around the meat and add the brown sugar, tomato paste, vinegar, mustard, soy sauce, bay leaves and paprika. Stir, then cover and cook on low 8 hours or on high 6 hours. Transfer the meat to a cutting board and let rest 10 minutes, then thinly slice.
Serve on brioche halves with coleslaw; drizzle with the cooking liquid.
You'll have plenty of leftover brisket: Use it in tacos, quesadillas, chili or even on a pizza.