How to thin a sauce
At times you may find that you just don't have enough liquid in your dish. Maybe your oven was a little high, maybe your pot was bubbling a little too vigorously, maybe your onion was too small or maybe you left something out.
Thinning a sauce isn't rocket science. Basically you just add more liquid – water, stock, juice, wine, milk or cream – about a tablespoon at a time and stir well to combine. The liquid you add should be determined based on what is already included in the recipe and you might like to use a combination.
If you're trying using a new appliance, are new to cooking or are just plain paranoid, it's best to check on your dish regularly the first few times you cook so you can start to get a feel for how it all works and what your sauce is doing. If you're cooking on the stovetop a quick glance should identify if the sauce is getting too thick. However, for oven cooking you'll need to leave it covered and untouched for at least 45 minutes (chicken) or 1 hour (meat) to give the veges a chance to start breaking down or you may start throwing in extra water unnecessarily.
The catch with thinning a sauce is it can affect the flavour balance. Refer to How to correct seasonings for some easy solutions.