Bring roughly chopped potatoes to a boil until it is tender and breaks apart when a fork is pierced through.
Strain out the potatoes from the water. While it is still hot, transfer in a food processor (or on a mixing bowl if you are using an immersion blender / masher / ricer). Pulse on high speed until potatoes are smooth and creamy. Bring in a mixing bowl.
Season the potatoes and add the butter, milk, garlic and onion powder, paprika, salt & pepper. Fold all ingredients on the potatoes until well combined & smooth.
Grab the casserole dish with meat filling. Pour mash potatoes on top of the meat filling and spread out into an even layer and smooth out the surface with a back of a spoon or spatula.
This part is optional (and mainly for aesthetics), using a back of a spoon (or spatula), create patterns or textures on the top layer of the mash potato. You can also run a fork to create streaks. This can also help baking the dish evenly.