1. Add the brisket to the inner pot. Add 4 cups of water to the pot, or just enough water to cover the brisket. Add the garlic, mustard seed, and black peppercorns.
2. Cover, lock the lid, and flip the steam release handle to the sealing position. Select Meat Stew (High) and set the cook time for 50 minutes.
3. While the brisket is cooking, preheat the oven to 170°F. Place a large ovenproof serving platter in the oven to warm for 8 minutes.
4. When the cook time is complete, allow the pressure to release naturally (about 20 minutes). Open the lid and transfer only the brisket to the warmed platter.
5. Add the onion, celery, and cabbage to the pot.
6. Cover, lock the lid, and flip the steam release handle to the sealing position. Select Soup (High) and set the cook time for minutes.
7. When the cook time is complete, quick release the pressure. Open the lid, add the brisket back to the pot and let warm in the pot for 5 minutes.
8. Transfer the warmed brisket back to the platter and thinly slice. Use a slotted spoon to transfer the vegetables to the platter. Serve hot.
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