This is an excellent tactical use of the microwave oven, and doesn’t need a recipe per se.
Put your berries in a microwave proof casserole. No water is needed, just sprinkle very lightly with caster sugar.
Just as a guide: for about 300g of frozen berries, I use a maximum of a tablespoon of caster sugar – perhaps a touch more if there are the tart varieties (like cranberries) in the mix, but also less for sweeter fruit (such as blueberries – we leave out the sugar entirely).
Microwave for 2 minutes, stir and go again. It really won’t take any longer for the berries to defrost and drop their juice. Don’t worry if you overdo it and the fruit disassembles itself – call it a coulis and go eat it anyhow! It is sumptuous just as is, but it is also a perfect topping for muesli, pancakes, french toast or whatever else takes your fancy.
PS: The Australian climate is generally not friendly to berries, so they are often quite pricey, but many wholesale outlets now do frozen berries in larger quantities that are quite economical. Also, our nearest “non-mainstream” supermarket has a local supplier who does a terrific 1kg bag of mixed red berries – sensational taste and value, locally grown to the fairly stringent standards required for certification in Australia. So it is probably worthwhile to shop around if your mainstream supermarket stocks only those that cost a king’s ransom.