Wash the russet potato and dice it into small, uniform 1/2-inch cubes to ensure even cooking.
Dice the red bell pepper and yellow onion into similar sized pieces, and finely chop the fresh parsley.
Cut the sirloin steak into bite-sized cubes, removing any excess fat, and season with half of the salt, pepper, and garlic powder.
Heat the olive oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the potato cubes to the skillet and cook for 8-10 minutes, stirring occasionally, until they are tender and develop a crispy golden crust.
Push the potatoes to the side of the pan and add the steak cubes, searing them for 2-3 minutes until browned on all sides.
Add the bell peppers and onions to the pan, stirring everything together with the remaining seasonings.
Sauté for another 4-5 minutes until the vegetables are tender-crisp and the steak is cooked to your preferred level of doneness.
Garnish with fresh parsley and serve immediately while hot and fragrant.