Season the chicken breast with half of the sea salt, black pepper, and dried oregano.
Grill or pan-sear the chicken over medium-high heat until the internal temperature reaches 165°F, then let it rest for 5 minutes before slicing into strips.
In a small bowl, whisk together the extra virgin olive oil, lemon juice, and the remaining salt, pepper, and oregano to create the vinaigrette.
In a large mixing bowl, combine the baby spinach, diced cucumber, halved cherry tomatoes, and thinly sliced red onion.
Add the cooked quinoa to the vegetables and drizzle with the lemon-oregano vinaigrette, tossing well to ensure everything is evenly coated.
Transfer the salad to a plate, top with the sliced grilled chicken, and sprinkle with crumbled feta cheese.