To make the sweet potato purée: Peel and cube 1 large sweet potato or 2 small sweet potatoes, then place them in a large pot of boiling water. Boil until soft, 15–20 minutes. Drain the water, and place sweet potatoes back in the pot. Use an immersion blender or potato masher to purée the sweet potatoes. This step can be done in advance. It should keep in the fridge for about 2 weeks. You can also freeze it for up to 6 months.
To make a flax egg: Whisk together 3 Tbs. warm water and 1 Tbs. flaxseed meal; place in the fridge for 1 minute to thicken.