Peel the beetroot and cut into small cubes. Mix together with all the ingredients for the marinade, put in an airtight container and let rest for at least 1 hour. It would even be better to marinate for 24 up to 48 hours in the fridge. Make sure to turn the container from time to time.
Preheat the oven at 220° Celsius.
Cook sushi rice according to the instructions on the packaging. I poured mine in a pan together with the water and salt, brought to boil, reduced the temperature to low, covered the pan and let simmer for 10 minutes. Then I added the garlic clove, removed the pan from the heat and let the rice rest for 15 minutes (pan covered).
While the rice is cooking, line beetroot cubes on a baking sheet (without marinade) and bake in the oven for about 20 minutes.
Peel ginger and grate it right into the cooked rice, mix well.
Arrange everything in a bowl and decorate with a sliced avocado half, cilantro leaves and sprinkle on some black sesame seeds and scallion rings.