A lot of wild meat can be gamey or otherwise tough, so old recipes for dealing with them often involved strong flavors like mustard or alcohol for flavoring or to esterify acids with alcohol and create more complex flavor. In this case, prepared mustard presents a significant source of acid as vinegar, and indeed some recipes call for a combination of white wine vinegar and white wine to assist.
Rabbit is a lean meat containing a high proportion of bone to muscle and which also has significant variety in flavor. Wild rabbits are typically smaller and gamier and the long cooking time helps to tenderize while the maillard reaction and esterification help to develop competing flavors. Potatoes both thicken and add bulk and cream helps develop texture. Rabbit is one of few meats I do not hesitate to add fats to. Google "rabbit starvation" to find out why.
You can do this with chicken too, just perhaps reduce the amount of butter and cream by half. —msmely
2-3 pound (if wild) or 3-4 pound (if farmed) rabbit (800 g - 1.2 kg or 1.5 - 2 kg)