Heat a tablespoon of oil over a high heat. Season the brisket with salt and pepper and sear until browned on all sides. Remove the brisket from the heat and set aside. Heat a second tablespoon of oil and cook the onions for 10 minutes until softened.
While the onions are cooking mix together the smoked paprika, mustard, final tablespoon of oil, cinnamon, cayenne, soy sauce, fish sauce, honey, red wine vinegar, garlic and sugar. Whisk until smooth.
Place the seared brisket and red onions into your slow cooker. Brush the brisket with the sauce mixture then pour the rest into the slow cooker over the brisket. Add the chipotle chillies, chopped tomatoes and bay leaves. Cook on low for 8-10 hours. Once done, pull the meat and stir through the sauce.
Nutrition information is an estimate and will vary depending on the exact ingredients used.