In a mixing bowl, combine olive oil, lemon juice, minced garlic, oregano, salt, and black pepper.

Place the chicken breasts in the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes.

In a saucepan, bring 2 cups of water to a boil. Add the rinsed quinoa, reduce heat to low, cover, and simmer for 15 minutes or until water is absorbed. Fluff with a fork.

Preheat the grill to medium-high heat. Remove chicken from marinade and grill for 6-7 minutes on each side or until the internal temperature reaches 165°F (75°C).

In a skillet, sauté mixed vegetables over medium heat for 5-7 minutes until tender.

Serve the grilled chicken breasts with sautéed vegetables and quinoa on the side.