Slow Cooker BBQ Beef Brisket

For the brisket:

• 1.5-2 kg brisket (3-4 lb)
• 1 tablespoon canola oil
• 1 large onion, sliced

For the spice rub:

• 1 tablespoon brown sugar
• 1.5 teaspoon paprika
• 1 teaspoon garlic powder
• 1/2 teaspoon cumin powder
• 1/2 teaspoon mustard seeds or dry mustard powder
• 1 teaspoon salt
• 1/2 teaspoon black pepper

For the bbq sauce:

• 1 carton tomato paste (200 g or 8 oz)
• 2 tablespoon Dijon mustard
• 2 tablespoon apple cider vinegar
• 1 teaspoon brown sugar
• 1 heaped teaspoon chili powder NOT cayenne pepper, see notes
• 1 heaped tsp paprika
• 1 teaspoon salt
• 1/2 tsp black pepper
• 2/3 cup water