Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Scrub the russet potato thoroughly and cut it into uniform 1/2-inch cubes, then pat them dry with a paper towel to ensure they crisp up.
Slice the chicken breast into even 1-inch strips or bite-sized pieces to ensure they cook at the same rate as the potatoes.
In a large mixing bowl, combine the chicken and potato cubes with the olive oil, dried rosemary, garlic powder, sea salt, and black pepper.
Toss everything together until the chicken and potatoes are well-coated in the oil and spices.
Spread the mixture in a single layer on the prepared baking sheet, making sure not to crowd the pan so the ingredients roast rather than steam.
Roast in the center of the oven for 20-25 minutes, flipping the potatoes halfway through, until the chicken is cooked through and the potatoes are golden brown.
Remove from the oven and immediately squeeze the juice of half a lemon over the hot chicken and potatoes.
Garnish with freshly chopped parsley and serve warm.