To make the crema, stir together the sour cream, crumbled feta, and milk. Refrigerate until ready to use.
Preheat the oven to 450 degrees. Bring a large pot of water to a boil. Salt the water heavily (I used 1/4 cup kosher salt) and add the potatoes.
Boil the potatoes for 20 minutes and drain. Place your fat of choice on a baking sheet and place in the oven to preheat.
While the fat is preheating, smash the potatoes to 1/2-inch thickness using a heavy skillet or the bottom of a glass.
Place the potatoes on the preheated baking sheet. Brush the tops of the potatoes with some of the oil from the baking sheet. Sprinkle with salt and place in the oven for 15 minutes. Turn the potatoes and return to the oven for another 15-20 minutes, until golden brown.
Remove the potatoes to a plate, sprinkle with parsley if desired, and serve immediately with the feta crema.