Wash the sweet potato and poke some holes in it with a fork. Place it on a baking sheet or a piece of foil, and bake until tender, 40 to 45 minutes. Remove it from the oven, and when it's cool enough to handle, remove the skin and slice it into long, skinny pieces.
Make the sushi rice while the sweet potato cooks.
To make the rice in the Instant Pot: Combine the rice and water in the Instant Pot. Stir to combine, then lock the lid and cook for 5 minutes with a 10-minute natural release. Stir in the rice vinegar and agave nectar, then transfer to a bowl to cool.
To make the rice on the stove: Bring the water to a boil in a saucepan on high heat, then add the rice. Cover the pan, reduce the heat, and simmer for 18 to 22 minutes. Remove the pan from the stove, but leave the lid on and let the rice steam for another 10 minutes. Stir in the rice vinegar and agave nectar, then transfer to a bowl to cool.
Make the sweet potato sushi.
Fill a small bowl with water. Grab your first nori sheet, and place it shiny side down on a dry surface. Wet your fingers in the water, and use them to spread the sushi rice all over the nori sheet, leaving about a 1" strip uncovered along one side.
On the side opposite the 1" strip, arrange a strip of sweet potato, avocado, and green onion. You want the ingredients to go all the way to the edge of the roll.
It's time to roll! Start on the side with the filling, and gently roll toward the 1" uncovered strip of nori, pressing gently as you go. If you have a sushi mat, you can use it, but you can roll without one. When you get to that uncovered bit of nori, dip your fingertips into the bowl of water, and sprinkle it onto that part of the nori. Gently finish rolling, pressing lightly as you roll back and forth to make a seal. Use a sharp knife to slice into pieces.
Repeat with the remaining pieces of nori and filling ingredients. Serve immediately.
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Notes
Leftover sweet potato sushi will keep for 2 to 3 days in the fridge, but do know that the sushi rice will harden in the refrigerator. Sushi is best eaten the day you make it.