Toast sesame seeds: In a dry pan over medium heat, toast sesame seeds until fragrant for about 2 minutes. Stir constantly to prevent burning.
Grind & mix: Using a mortar and pestle (or suribachi if you have one), grind the sesame seeds until they release their oils and form a coarse paste. Add soy sauce, honey and mirin or rice vinegar; mix until smooth and aromatic.
Blanch spinach: Bring a pot of water to a boil with salt. Add spinach and blanch for 30–45 seconds, just until wilted.
Cool & squeeze: Immediately transfer to ice water to stop the cooking. Gently squeeze out excess water and cut into 2-inch lengths.
Dress & serve: In a bowl, toss spinach with the sesame dressing until evenly coated. Arrange neatly on a small plate.