I set the double oven(I think) to 150 and left thinly sliced courgettes in there for about 20 minutes.
The skin of the courgette looked a bit dark to me and it tasted a bit dry:
How can one know if it is overcooked and how can I avoid this?
Are these courgettes overcooked, if so how can I avoid it
overcooking
Related Solutions
Most ricer cookers use temperature of the pan - which is regulated by water content! - to decide when to stop. Only when enough water has evaporated to leave you with reasonably dry, palatable grains can the pan temperature exceed 100°C/212°F. If what you are cooking cannot absorb all the water, the device will cook until all the water has evaporated (which the lid makes very slow).
If actually boiling it dry doesn't stop it, and no "keep warm" or "saute" or similar special mode has been selected, return it, it is broken.
What you get is something like a Hershey chocolate bar:
To someone who's only ever had Belgian or Swiss high-quality chocolate his entire life, and tries a Hershey Chocolate bar late in life, it'll taste "burnt" and not nice at all, whereas if that's what you grew up with, it'll taste "normal"...
As you're wondering why, you seem to have grown up with that taste, so you don't see what all the fuss is about, whereas any Belgian or Swiss national will not like your product (and why Hershey is not very popular in that region).
There is no way to fix overheated chocolate to the chocolate connoisseur except to throw it away and start anew.
Best Answer
I think the biggest issue is you're baking them instead of roasting them. In order to roast them, you'll need a much higher temperature. 230c or 450f is about right. As Joe pointed out, the pieces are a bit thin for roasting. Spears or large chunks will serve you better. Roast them skin side down and brushing on a little olive oil will prevent the skins from drying out.