Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Scrub the yellow potato and cut it into one-inch cubes, then place them on one side of the prepared baking sheet.
Mince the garlic clove and whisk it in a small bowl with one teaspoon of olive oil, the dried oregano, sea salt, black pepper, and the juice from the lemon half.
Pat the chicken thighs dry with a paper towel and place them on the other side of the baking sheet.
Drizzle the lemon-herb mixture over both the chicken and the potatoes, tossing the potatoes with a spatula to ensure they are evenly coated.
Roast for 25 to 30 minutes until the chicken reaches an internal temperature of 165°F and the potatoes are golden and tender.
While the chicken roasts, slice the cucumber into thin rounds.
In a medium bowl, toss the fresh baby spinach with the remaining half teaspoon of olive oil and the sliced cucumbers.
Serve the roasted chicken and potatoes immediately over the bed of fresh spinach and cucumber salad.