Cut the eringi mushrooms: trim the base, separate the cap and stem, slice the cap thinly, and score the stem with a crosshatch pattern. Cut mizuna into 2cm pieces and crush garlic, removing the core, then chop roughly.

Heat half the olive oil (1/2 tbsp) in a pan over medium heat and cook the scored eringi until browned. Remove from the pan.

Add the remaining olive oil (1/2 tbsp) and garlic to the pan, cook on low heat until fragrant. Add water, bring to a boil, then add spaghetti (broken in half), tuna (including oil), and salt. Cover and cook on medium-low heat, stirring occasionally, for the pasta's package cooking time.

Uncover, add the cooked eringi, and steam for 1-2 minutes with the lid on.

Add soy sauce, butter, and a pinch of salt and pepper. Stir-fry over medium-high heat briefly.