Do not overcook the chicken or it will be dry.
The chicken doesn’t need to cook through when you cook it in the pan, since it will finish cooking in the oven. All you are looking for is a good colour and crust.
Reducing the cooking liquid until it becomes a sauce will both concentrate the flavour and provides the proper thickness.
Traditionally, a chicken chasseur is a whole chicken cut up in leg meat and breast meat, that is stewed in the sauce for a long time until cooked. This recipe is a more modern version.