Bring a pot of water to a boil. Place sweet potatoes in a steamer over boiling water and cover. Let steam for 7 minutes, or until tender. Mash with a potato masher.
Meanwhile, heat oil in a medium skillet and saute the onion and garlic until soft. Set aside.
In a bowl, add black beans and mash with a potato masher. Mix in the sauteed onion and garlic, corn, bell pepper, water, mustard, cumin and tamari until thoroughly combined. Season with salt as needed.
Divide the mashed sweet potato and black bean mixture evenly between the tortillas and fold into burritos.
If eating immediately, heat the burritos in the oven at 350oF (177oC) for 10 to 12 minutes or until warmed through. Wrap the remaining burritos in foil and freeze in a freezer-safe bag. See notes section for instructions on how to reheat.