Rinse rice off in a colander about 5 or 6 times, then place in a sieve and let it drain over a bowl for 45 minutes. This step is important.
While waiting for the rice to drain, prepare sushi liquid seasoning by heating rice vinegar, mirin, salt and caster sugar and lemon juice in a small pot till sugar is dissolved. Don't boil.
After the rice has drained, add it and 1 1/3 cup of water to a heavy based small pot. Bring to a boil.
Once boiling, cover with a lid and reduce to a simmer. Let cook for 15 minutes exactly. Avoid opening the pot. Once 15 minutes is up, turn the stove off and leave the pot covered for 10 minutes.
Transfer rice to a bowl and pour sushi seasoning over it. Using a bamboo paddle or large spoon, with a cutting motion, stir/fluff seasoning into the rice. The cutting motion will help to aerate the rice.
Once cooled, the rice should be sticky and have absorbed all the liquid seasoning. It is now ready to use!
In a pan, place the Shimeji mushrooms, tamari, mirin and sesame oil over a small to medium heat. Let cook for a few minutes at most, stirring occasionally, till mushrooms soften and release their moisture into the pan - this liquid is what you'll use as your dipping sauce. Remove the mushrooms.
Place rice in a bowl, and surround with prepared mushrooms, carrot, avocado, tofu, cucumber. Sprinkle sesame seeds and seaweed flakes on the rice. Serve with dipping sauce!