In a large bowl, whisk together olive oil, lemon juice, red wine vinegar, minced garlic, oregano, paprika, salt, and pepper. Add the chicken breasts and toss to coat completely. Let marinate for 15 to 30 minutes at room temperature.

While the chicken marinates, prepare your grain base. Cook brown rice or quinoa according to package directions. Fluff with a fork and set aside to cool slightly.

Heat a grill or large skillet over medium heat. Cook the marinated chicken for 5 to 6 minutes per side, or until the internal temperature reaches 165°F. Let the chicken rest for 5 minutes before slicing.

Make the tzatziki by grating the cucumber, then pressing out excess water using a clean kitchen towel or cheesecloth. In a medium bowl, combine Greek yogurt, grated cucumber, lemon juice, minced garlic, and chopped dill. Season with salt and pepper. Refrigerate until ready to serve.

Dice the cucumber, halve the cherry tomatoes, thinly slice the red onion, and pit and halve the olives.

To assemble, layer your bowls starting with the grain base. Top with sliced chicken, chopped vegetables, a generous spoonful of tzatziki, crumbled feta, and fresh parsley. Add lemon wedges on the side and any extras like avocado slices.