Add all the ingredients to the Thermomix bowl. Mix for 3 sec/speed 4.
Cook for 20 min/Varoma/speed 1, without the measuring cup. There's no need for the simmering basket or splatter guard, as the volume of liquid is small.
Transfer to a heatproof jar or container and cool completely before using. The sauce thickens slightly as it cools, to the consistency of maple syrup.
Notes
Mirin and cooking sake: Both are sold in the Asian aisle at most major supermarkets.Storage: Store the teriyaki sauce in an airtight jar or container in the fridge for up to 2 weeks. Give it a stir before using if it has been sitting for a few days.