Cut the potatoes into ½-inch cubes or thin slices, depending on your preference. Dry the cut potatoes thoroughly with paper towels to remove excess moisture.
Place cut potatoes in a large bowl. Sprinkle with garlic powder, paprika, dried thyme, salt, and pepper. Toss until evenly coated.
Heat a large cast-iron skillet over medium-high heat. Add bacon fat or oil and butter.
Add diced onions to the hot skillet and sauté for 2-3 minutes until they begin to turn translucent.
Add the seasoned potatoes to the skillet, spreading them in an even layer. Leave undisturbed for 4-5 minutes to develop a golden crust.
Flip the potatoes using a spatula and continue cooking for about 20 minutes, turning occasionally to ensure even browning.
Taste and adjust seasonings if needed. Remove from heat and garnish with freshly chopped parsley.