Marinate chicken:
Combine olive oil, lemon juice, minced garlic, oregano, salt, and pepper in a bowl using a whisk. Add chicken breasts and thoroughly coat with the mixture. Allow to marinate for at least 30 minutes.Grill chicken:
Preheat a grill pan or skillet over medium-high heat. Take chicken out of the marinade and cook each side for 6-7 minutes, until fully cooked and internal temperature hits 165°F (75°C).Cook quinoa:
During chicken cooking, prepare quinoa or rice as per package directions. Once done, reserve.Chop vegetables:
Cut cucumber into dice, halve cherry tomatoes, and slice red onion thinly. Transfer vegetables to a large bowl.Assemble bowls:
Place a bed of quinoa or rice in each serving bowl. Slice the cooked chicken and arrange on top.Add vegetables:
Distribute the chopped vegetables evenly around the chicken in each bowl.Top with feta:
Sprinkle crumbled feta cheese and chopped parsley over each bowl.Add tzatziki:
Add a generous spoonful of tzatziki sauce on top or offer it on the side for dipping.