Cloud inversions, also known as temperature inversions, form in valleys when temperatures near the ground are lower than higher in the air.
This certainly is not an everyday occurrence and the best time of year is Autumn or Winter. To be in with any catch of catching one, you need to climb to the top of a hill that ideally overlooks a lake and simply try and get above the clouds.
Typically this happens as the sun comes up, water evaporates from the surface of the lake, but starts to condense as it rises and hits colder air forming mist. This can form thick low clouds if it becomes trapped and cannot rise from underneath warmer air that has been heated by the sun from above.
So you need to check the temperature and humidity levels at different elevations. If the temperature closest to the ground is colder than the temp at a higher elevations and humidity is high, it's likely a cloud inversion could happen.
The dew point ideally needs to be lower or the same as the forecasted temperature for mist to form. There are a couple of apps that are good for checking all of this: Ventusky & Clear outside.
The best cloud inversions I’ve ever seen have been from Ben Nevis. However Snowdonia and the Lake District are great places to try too. Often from the summit of Helvellyn, as it sits way above both Thirlmere and Ullswater.