Add the garlic, soy, mirin, sake, and sugar to the Instant Pot insert. Nestle in the chicken thighs skin side up. Turn the Instant Pot on to High Pressure and cook for 7 minutes.When the Instant Pot is done cooking, quick release the pressure. Carefully open the lid and remove the chicken thighs onto a plate.Turn the sautè function onto medium and add the cornstarch and water mix to the the sauce in the insert. Stir and let simmer and thicken to your liking. Turn off the Instant Pot and spoon sauce onto the thighs. Top off with green onions and toasted sesame seeds, if using. Enjoy!