Scottziki Sauce
1/2 cup plain yogurt
1/2 cucumber - peeled, seeded and finely chopped
1/2 teaspoon chopped fresh dill, or to taste

Sandwiches
2 tablespoons olive oil
3 large cloves garlic, minced
1 large red onion, thinly sliced
1 pound ground beef
1 pinch dried oregano, or to taste
2 stalks celery, chopped
1 bunch fresh spinach, cleaned and stemmed
1/2 cup sliced fresh mushrooms (optional)
5 pita breads, warmed
1/2 cup banana pepper rings
2 tomatoes, cut into wedges