1.Rinse and drain brown rice, place into a saucepan over medium heat, and pour in water. Bring to a boil, and simmer until rice has absorbed the water, about 45 minutes. Add rice vinegar and sugar to cooked brown rice. Mix well and set aside.
2.Cut carrot and cucumber into 8 long thin strips, each. Cut seeds off cucumber strips.
3.Cut avocado into half, remove skin and pit, and cut each half into 8 slices.
4.Place nori sheet horizontally in front of you. Spread rice evenly on 2/3 of nori sheet; lay a few leaves of spinach or a small amount of sprouts in the bottom third of the rice. Place 2 cucumber strips, 2 carrot strips, and 4 pieces avocado on top.
5.Slightly dampen the top edge of the nori. Starting from the bottom, roll up tightly. Press the damp edge to seal.
6.Cut into thick pieces and sprinkle with sesame seeds, if desired. Enjoy with or without soy sauce on the side.