Dilute the mentsuyu noodle sauce as you would for making a dipping sauce for noodles. Prepare an amount that will be enough to cover the French fries.
Put the diluted mentsuyu noodle sauce into a pot and bring to a boil. Add the French fries and simmer for 1 minute.
Turn off the heat and wait about 2 minutes for the French fries to absorb the flavors. Strain the liquid out with a strainer or pour the liquid out while you hold the lid on the pot.
Transfer to a plate and it's done. It's good cold too, so you can pack it in bentos.
Story Behind this Recipe
Once in a while, I go to McDonald's. If the service is really welcoming, I somehow end up ordering a large French fry. But I can't finish them, and I don't really like eating them cold the next day...So, I tried simmering them.
It seems like a waste to pour out the mentsuyu sauce, but it actually removes excess oil and makes the fries a little healthier!