In a medium mixing bowl, whisk together olive oil, lemon juice, minced garlic, dried oregano, salt, and black pepper until well combined. Add the chicken breasts, ensuring they are fully coated in the marinade. Cover the bowl with plastic wrap and let the chicken marinate in the refrigerator for 20 minutes.

While the chicken marinates, prepare the tzatziki sauce. Grate the cucumber and squeeze out any excess moisture. In a bowl, combine the drained cucumber with Greek yogurt, minced garlic, lemon juice, chopped dill, and a pinch of salt.

Heat a large skillet over medium-high heat and add a drizzle of olive oil. Once hot, add the marinated chicken breasts and cook for 3-4 minutes per side or until golden brown and cooked through. Remove the chicken from the skillet and let it rest for a minute before slicing.

Prepare your taco toppings by halving the cherry tomatoes, slicing the red onion, crumbling the feta cheese, and rinsing the fresh spinach or arugula.

To warm the tortillas, heat a dry skillet over medium heat and place the tortillas for about 20 seconds on each side until they become warm and pliable.

Assemble the tacos by spreading tzatziki on each warm tortilla, layering on the sliced chicken, topped with cherry tomatoes, red onion, feta, and greens.