Whisk together olive oil, lemon juice, minced garlic, oregano, chopped parsley, dill, red wine vinegar, salt, and pepper in a bowl. Place chicken in a large zip-top bag or container. Pour marinade over chicken and massage to coat evenly. Refrigerate at least 2 hours, preferably overnight for maximum flavor.
Grate cucumber using a box grater. Place grated cucumber in a kitchen towel or cheesecloth and squeeze hard to remove all excess water. This step is essential or your tzatziki will be watery and thin instead of thick and creamy.
In a medium bowl, combine Greek yogurt, squeezed-dry cucumber, minced garlic, lemon juice, chopped dill, olive oil, and salt. Stir until well combined and smooth. Cover and refrigerate while preparing other components. Flavors improve as it sits.
Prepare your grain base according to package directions. For basmati rice, use 2 cups uncooked rice with 3 cups water. For quinoa, use 2 cups quinoa with 3 cups water or broth. Add a pinch of salt and drizzle of olive oil. Cook until tender and fluffy, then let cool completely before assembling bowls.
Heat your grill or grill pan to medium-high heat (about 400°F). Lightly oil the grates to prevent chicken from sticking. Let it get properly hot before adding chicken for best sear marks and flavor.
Remove chicken from marinade, letting excess drip off. Place on hot grill and cook 6-7 minutes per side until internal temperature reaches 165°F and chicken has nice char marks. Don't move chicken around too much or you won't get those pretty grill marks. Let rest 5 minutes.
Transfer grilled chicken to a cutting board and let rest for 5 minutes to allow juices to redistribute. Slice chicken against the grain into thin strips or bite-sized pieces. Slicing against the grain keeps chicken tender and easier to eat in the bowls.
While chicken rests, dice English cucumber into small cubes. Halve cherry tomatoes. Thinly slice red onion (soak in cold water for 5 minutes to mellow the bite, then drain). Pit and halve Kalamata olives. Crumble feta cheese. Chop fresh parsley or dill for garnish.
For each bowl, start with a generous base of cooked rice, quinoa, or couscous (about ¾ to 1 cup per bowl). Arrange sliced grilled chicken on one side. Add portions of diced cucumber, halved tomatoes, sliced red onion, and olives around the bowl in sections for a beautiful presentation.
Sprinkle crumbled feta cheese generously over the entire bowl. Drizzle tzatziki sauce over everything or serve it on the side for dipping. Garnish with fresh chopped parsley or dill. Finish with a drizzle of olive oil and squeeze of fresh lemon juice if desired.
For meal prep, store all components separately in airtight containers. Keep chicken in one container, cooked grain in another, chopped vegetables together, and tzatziki in its own jar. Feta should be kept separate too. Everything stays fresh for 5 days this way. Assemble bowls fresh each day for best texture and flavor.