COMBINE all the octopus ingredients in a bowl, then place in a tall pot. Cover with tepid water.
BRING pot to a simmer and poach octopus until it's tender, around one hour.
REMOVE the octopus from liquid and chill.
REMOVE the tentacles and grill until warmed through.
BOIL the potatoes in salted water until tender. Remove potatoes from the water and chill.
SMASH the potatoes with the palm of your hand and fry potatoes in 375°F oil, in large tall-walled pot, until golden brown and crispy.
PLACE potatoes on a tray that has been lined with paper towels to absorb the excess oil. Season with sumac and salt.
REMOVE sliced red onions from water and pat dry. Combine the red onion and the rest of the slaw ingredients in a bowl. Marinade at room temperature for at least one hour.
SEASON with salt.
COMBINE all tahini ingredients in a blender and blend until smooth. If the mixture is too thick, add some water until the tahini reaches a smooth and creamy consistency.
SEASON with salt.
PLACE a spoonful of tahini at the bottom of the plate then top with the crispy potatoes, octopus and the slaw to finish. To make it easier to eat, slice the tentacles in thin rounds.