Grate half a cucumber and squeeze out as much moisture as possible using a clean kitchen towel or paper towels to ensure a thick sauce.

In a small bowl, combine the strained cucumber with Greek yogurt, minced garlic (1 clove), lemon juice, chopped dill or mint, and a pinch of salt. Stir well, cover, and refrigerate while preparing the beef.

Heat olive oil in a large skillet over medium-high heat. Add the finely diced onion and sauté for about 3 minutes until softened and translucent.

Add the ground beef to the skillet. Break it up with a wooden spoon and cook until fully browned with no pink remaining. Drain any excess fat if needed.

Reduce heat to medium-low. Stir in minced garlic (2 cloves) and tomato paste. Cook for about 60 seconds until tomato paste darkens slightly.

Sprinkle in dried oregano, cumin, coriander, and paprika. Stir for 30 seconds until fragrant.

Pour in red wine vinegar or lemon juice. Season with salt and black pepper to taste. Simmer for a final minute.

While beef simmers, warm pita bread by lightly charring in a dry skillet or toasting in the oven.

Spoon a generous portion of the hot spiced ground beef into the center of each warmed pita.

Top with tzatziki sauce, sliced tomatoes, thinly sliced red onion, crumbled feta cheese, and chopped parsley or cilantro. Fold and serve immediately.