In a medium bowl, whisk together olive oil, lemon juice, minced garlic, dried oregano, salt, and black pepper until well combined. Add the chicken and let marinate for at least 20 minutes.

While the chicken marinates, prepare the tzatziki sauce by mixing grated cucumber, Greek yogurt, minced garlic, chopped dill, and a pinch of salt.

Cook the marinated chicken in a skillet over medium-high heat for 3-4 minutes on each side until golden brown, then let rest before slicing.

Slice the cherry tomatoes and red onion, crumble the feta, and wash the spinach or arugula. Arrange toppings on a platter.

Warm the tortillas in a non-stick skillet for about 15-30 seconds on each side or microwave for about 10 seconds wrapped in a damp towel.

Assemble tacos by spreading tzatziki sauce on tortillas, layering on chicken, tomatoes, onion, feta, and greens.