Heat the grill to high. Add the spinach, butter, water and garlic to a blender and blitz until vivid green, 1 minute or so.
Rinse the sushi rice under cold water a few times until it runs clear, then drain. Add to a small saucepan with the spinach water and bring to the boil. Once boiling, cover with a tight-fitting lid, turn the heat down to low and cook for 12 mins. Turn off the heat and leave to steam (lid on) for another 12 mins.
In a mixing bowl, whisk together the miso, honey, soy sauce, lemon juice, MSG (if using), a pinch of salt and some black pepper. Slice each chicken thigh into three strips, then toss through the miso marinade.
On a tray, lay out the chicken and spoon over any extra sauce. Grill for 6–8 mins until charred and cooked through.
Serve the spinach rice in bowls, top with the grilled chicken and spoon over any sauce from the tray. Finish with an egg yolk, chopped chives and a wedge of lemon.
