In a small bowl, whisk together the lemon juice, minced garlic, dried oregano, sea salt, and black pepper to create the marinade.
Place the chicken breast in a shallow dish and coat with the marinade; let it sit for at least 15 minutes for the flavors to penetrate.
Preheat a grill or grill pan over medium-high heat and cook the chicken for 6-7 minutes per side until the internal temperature reaches 165°F.
While the chicken rests, dice the cucumber and red onion, and halve the cherry tomatoes.
In a large mixing bowl, combine the cooked quinoa, diced vegetables, and crumbled feta cheese.
Drizzle the extra virgin olive oil over the quinoa mixture and toss gently to combine.
Slice the grilled chicken into strips and serve it warm over the chilled quinoa salad for a perfect temperature contrast.