Mince the onion and slice the mushrooms.

Heat olive oil in a pan over medium heat, add onion and sauté until softened. Add the mackerel (including the oil from the can) and break it apart while cooking. Add mushrooms and continue sautéing.

Add the washed rice and stir for about 2 minutes until translucent. Add the seasonings (sake, salt, pepper, and water) and bring to a boil. Flatten the rice, cover, and cook on low heat for 12 minutes.

Increase to medium-high heat and cook for about 30 seconds until you hear crackling sounds. Turn off the heat and let it steam for 10 minutes. Remove the lid and gently mix the rice.