Prep the ingredients per the instructions above.
Mix the water and cucumber together. Refrigerate until ready to serve.
Make the miso soup: Place the miso into a small bowl, add a little hot water, and whisk until smooth. (This will ensure it doesn’t clump.) Place all ingredients in a medium saucepan and bring to a low simmer. Refer to the directions on the package for cooking the kelp. Once done, keep covered with a lid to keep warm. Reheat if needed before serving.
Prepare the rice according to the directions on the package. Once cooked, set rice aside in a mixing bowl and season with salt and rice wine vinegar to taste. Allow rice to cool to room temperature.
Season the beef well with salt, pepper, and soy sauce.
While the rice is cooling, heat the oil in a skillet on high heat. Saute the beef until nicely browned then remove from the skillet. Season with salt and pepper to taste.
Dampen your hands with water (to prevent the rice from sticking), portion, and spread the rice on the nori sheets, covering as much of the nori as possible evenly. (Leave about 1 inch along the top and bottom of the nori sheet uncovered, as this will be how you seal your roll. Carefully flip the nori over onto a bamboo sushi mat or (a piece of plastic wrap on top of a towel) so that the rice side faces down and the nori side faces up.)
Lay the beef and avocado across the middle (use what's needed for a full roll, try not to overfill or the nori may rip when rolling). Slowly curve the bamboo mat over the filling and gently press along the length of the "sushi roll." (This motion presses the rice and the filling together and prevents the roll from being too loose). Continue to roll the sushi until you've rolled past the far edge of the nori sheet. Once rolled, gently press and form the length of the mat to make sure the sushi roll is compact and tight.
Slice the sushi roll into 6-8 pieces (you may have to rinse or wipe the blade in between slices to prevent sticking and tearing).
Top the sushi with crispy fried onion and scallions, and drizzle with teriyaki sauce. Serve with soup and cucumber water.