Biologically appropriate raw food (BARF) diets are probly the best food you can give to your ferrets because you are cooking for your ferret so you really know what you are giving to your ferrets.
The are many kinds of raw diets
Here's an example that I found in the book "Ferrets" by Vickie McKimmey :
"Kerry's Chicken Gravy"
2 fryers chickens (with giblets)
2 cups chicken livers
double handful, about half-pound, of fat trimmings (1 use chicken, but you can use beef or pork) or use a small canister/block of lard. Necks and backs can be added as well.
2 cups water
2 to 3 tbsp. Ferretone
2 to 3 tbsp. light olive oil
2 tubes Nutri-Cal or Nutri-Stat
6 tbsp. honey
Optional: A whole rabbit can be added to the mix, just increase the water to 3 cups. You can also add chicken hearts and gizzards or necks and backs, If you are trying to entice a ferret to try the gravy, you can add ground kibble to the mix to add a familiar smell. After they have accepted the gravy as food, you no longer need to add the kibble.