When it comes to retaining the flavour of dried herbs, dehydrating them in the microwave is the best way. But for a fresh, bright flavour in sauces and soups, freezing herbs in oil is key, and freezing them flat is the way to go.
We've always liked to freeze herbs by chopping them and transferring them to an ice cube tray with oil. Serious Eats managing culinary director J. Kenji López-Alt conducts some extensive tests on frozen herbs, discovering that there's an even more efficient way and flavourful way to do the same thing.
To preserve herbs in the freezer, blanch them first in water, then dry and chop them, add in a neutral oil, and transfer the mixture to a zip-locked plastic bag. The aha moment comes during storage: spread the bagged herb mixture in a thin layer, leave a centimetre or so for headspace, then freeze that flat bag on a baking sheet.
Herbs stored this way are much easier to use than their ice cube counterparts: simply cut off what you need with kitchen scissors, and store the rest back in the freezer. In addition, herbs packaged this way will freeze (and, conversely, melt) much quicker than their cubed counterparts, meaning they're less likely to overcook and lose their fresh flavour.
Freeze Fresh Herbs for Long-Term Storage [Serious Eats]