Marinate the Chicken: In a medium bowl or a large zip-top bag, combine the chicken breasts with the marinade ingredients (olive oil, lemon juice, oregano, garlic, salt, and pepper). Mix well to coat. Let it marinate in the refrigerator for at least 30 minutes or up to 4 hours.
Cook the Quinoa: While the chicken marinates, cook the quinoa. In a medium saucepan, combine the rinsed quinoa and 2 cups of water or broth. Bring to a boil, then reduce heat to low, cover, and simmer for 15-20 minutes, or until the liquid is absorbed. Fluff with a fork.
Cook the Chicken: Heat a large skillet over medium-high heat. Remove the chicken from the marinade and cook for 5-7 minutes per side, until golden brown and cooked through (internal temperature reaches 165°F / 74°C). Let the chicken rest for 5 minutes before dicing into bite-sized pieces.
Make the Cucumber Salad: In a medium bowl, combine the diced cucumber, cherry tomatoes, and red onion. Drizzle with olive oil, lemon juice, and oregano. Season with salt and pepper and toss to combine.
Assemble the Bowls: To build your bowls, start with a base of quinoa. Top with the diced Greek chicken, a generous scoop of the cucumber salad, a dollop of hummus, a spoonful of tzatziki, and a sprinkle of feta cheese and olives.