In a mixing bowl, whisk together olive oil, lemon juice, minced garlic, dried oregano, salt, and black pepper. Add the boneless, skinless chicken breasts and coat thoroughly. Cover and refrigerate for at least 30 minutes.

Preheat grill or skillet over medium heat for about 5 minutes. Remove marinated chicken, let excess marinade drip off, and cook each side for 6-7 minutes until the internal temperature reaches 165°F (74°C). Let the chicken rest before slicing.

Prepare your base by cooking quinoa or rice as per package instructions, typically combining 1 cup of grain with water, boiling, then simmering covered for 15-20 minutes until tender. Fluff with a fork.

Chop fresh vegetables: dice the cucumber, halve the cherry tomatoes, and thinly slice the red onion. Finely chop fresh parsley if using.

In a bowl, start with cooked quinoa or rice as the base, arrange sliced marinated chicken on top, and add chopped vegetables. Sprinkle crumbled feta cheese and drizzle with tzatziki sauce.

Garnish with freshly chopped parsley and serve immediately. For meal prep, store components in separate airtight containers.