In a small bowl, combine olive oil, dried oregano, and salt and pepper. Rub over the chicken breasts until evenly coated. Let marinate at room temperature for 30 minutes, or cover and refrigerate for up to 2 hours for more flavor.

Prepare 1 cup quinoa or rice according to the package directions. Fluff with a fork and keep warm.

Heat a grill pan or skillet over medium-high heat. Cook the chicken 5–7 minutes per side until an instant-read thermometer reads 165°F. Remove from the pan and let rest for 5 minutes before slicing thinly against the grain.

Stir together Greek yogurt, lemon juice, minced garlic, and a pinch of salt and pepper. Taste and adjust lemon or garlic as needed. Chill briefly if you like it cold.

Dice cucumber, bell pepper, and red onion, and halve the cherry tomatoes. Toss lightly with a pinch of salt if you want extra brightness.

Divide quinoa or rice between bowls, top with sliced chicken, arranged vegetables, crumbled feta, and a generous dollop of tzatziki. Finish with chopped fresh parsley for color and aroma.