In a mixing bowl, whisk together olive oil, lemon juice, garlic, oregano, salt, and pepper. Add chicken, ensuring it's coated. Marinate for at least 20 minutes.

Grate the cucumber and drain excess moisture. Combine with Greek yogurt, minced garlic, lemon juice, dill, and salt in a bowl. Mix well and chill.

Heat a skillet over medium-high heat, add olive oil, then sear marinated chicken for 3-4 minutes per side until cooked through. Let rest and slice.

Chop toppings: dice cherry tomatoes, slice red onion, crumble feta cheese, and wash spinach.

Heat tortillas in a skillet for 20 seconds on each side until warm, or microwave wrapped in a damp towel for 10 seconds.

Assemble tacos: Spread tzatziki on tortillas, add chicken, toppings, and more tzatziki if desired. Serve immediately.