In a medium bowl, whisk together the lemon juice, half of the olive oil, dried oregano, garlic powder, sea salt, and black pepper.
Add the chicken breast to the bowl and toss to coat thoroughly; let it marinate for at least 10 minutes.
Heat the remaining olive oil in a non-stick skillet over medium-high heat.
Place the chicken in the skillet and cook for 5 to 7 minutes per side, or until the internal temperature reaches 165°F and the exterior is golden brown.
While the chicken cooks, combine the diced cucumber, cherry tomatoes, and red onion in a small mixing bowl.
Remove the chicken from the pan and let it rest for 3 minutes before slicing into strips.
Assemble the bowl by placing the cooked quinoa at the base, then topping it with the sliced chicken and the fresh vegetable salad.
Garnish with crumbled feta cheese and serve immediately.