The priorities are:
1.) your inventory
2.) the fridge
3.) the mini fridges in the order that you place them.
Within each, the priority goes from the last slot (bottom right) to the first (top left).
I try to put a stack of cheap fish in the last slot of the fridge and pay attention to my inventory. Once you have several fridges, you can also place fish that you need for specific recipes in the last mini fridge and keep a stack of common fish in the kitchen fridge.