Place the spinach in a large pan over medium heat and cover until wilted.
In a small pan, heat the olive oil and saute mushrooms until soft. Keep both spinach and mushrooms warm.
Fill a small pan with water and umeboshi vinegar. Bring the water to the boil and crack the eggs into it to poach. Reduce the heat slightly and cook for approximately 3 minutes until the white is cooked and the yellow is still slightly runny.
Toast bread and serve eggs on top and spinach and mushrooms on the side.