Press the Saute button on an Instant Pot. Heat the oil, then add the onion, carrots, and celery and season with salt and pepper. Saute for 3-4 minutes, until just softened. Add the garlic and cook for 30 seconds. Press Cancel.
Add the broth, water, cabbage, tomatoes, thyme, bay leaves, and red pepper. Season with salt and pepper to taste. Secure and seal the lid and cook on Manual High for 5 minutes. Allow a 10-minute Natural Release, then release the remaining pressure.
Carefully remove the lid, then remove and discard the bay leaves and spent thyme sprigs. Taste and season if necessary, then serve garnished with parsley.
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Notes
Storing: You can store cabbage soup in an airtight container in the fridge for 5-6 days. The vegetables will become softer over time so it's best consumed earlier than later. Freezing: Add completely cooled soup to an airtight container or freezer bags for up to 4 months. Thaw in the fridge overnight before reheating and serving.