Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Scrub the russet potato thoroughly and cut into 1/2-inch cubes; dice the chicken breast into similar bite-sized pieces to ensure even cooking.
In a large mixing bowl, combine the potato cubes, chicken pieces, olive oil, minced garlic, sea salt, black pepper, and dried oregano.
Toss the mixture until every piece of chicken and potato is thoroughly and evenly coated in the oil and seasonings.
Spread the mixture in a single layer on the prepared baking sheet, making sure the pieces aren't crowded so they roast rather than steam.
Roast for 20 to 25 minutes, tossing halfway through, until the potatoes are golden-brown and the chicken is fully cooked.
Remove from the oven and immediately sprinkle the grated parmesan cheese over the hot chicken and potatoes.
Place the fresh baby spinach in a serving bowl and top with the warm roasted potato and chicken mixture to slightly wilt the greens before serving.