Preheat your oven to 400°F and line a small baking sheet with parchment paper for easy cleanup.
In a small mixing bowl, whisk together the olive oil, lemon juice, dried oregano, sea salt, black pepper, and minced garlic.
Place the chicken breast on the prepared baking sheet and brush thoroughly with half of the lemon-herb vinaigrette.
Roast the chicken for 18 to 22 minutes, or until the internal temperature reaches 165°F, then let it rest for 5 minutes before slicing into strips.
While the chicken rests, halve the cherry tomatoes and dice the cucumber into bite-sized pieces.
In a large serving bowl, combine the baby spinach and cooked quinoa, tossing them with the remaining half of the lemon-herb vinaigrette.
Arrange the sliced chicken, tomatoes, and cucumbers over the quinoa base and serve immediately.