Step 1

If oven cooking, preheat the oven to 300 degrees.

Step 2

In a large dutch oven (or skillet, if cooking in the slow cooker), heat the oil over med-high heat. Add the brisket to the pan fat side down and cook until golden brown, approximately 4-5 minutes. Turn the brisket over and brown the other side, approximately 5-6 minutes. Turn off the heat and drain the fat. (If cooking in the slow cooker, transfer the brisket to the slow cooker at this time.)

Step 3

Pour the ale over the brisket. Stir in the water, carrots, celery, mustard, thyme, mustard seeds and bay leaf. Cover and transfer to the oven for 3 hours. (Or cover and turn the slow cooker on low for 6-7 hours.)

Step 4

When the brisket has one hour left of cook time, heat the olive oil in a large skillet. Stir in the butter until melted. Add the onion and cook until softened. Add the cabbage. Cook until wilted and beginning to caramelize. Season with salt and pepper.

Step 5

Transfer the skillet to the oven and roast for 15-20 minutes stirring after 10 minutes. (The onions and cabbage should start to turn golden brown and caramelize.)

Step 6

When the brisket is done, it should be tender enough to pull apart with a fork. If it's still tough, continue to cook for an additional 30 minutes. Allow to rest for 10-15 minutes covered with foil before slicing against the grain. Serve with the cabbage.