Toss the chicken and fennel with the oil, garlic and a little seasoning. Preheat a griddle pan (or ordinary frying pan), then add the pitta breads and cook for 1–2 minutes, turning until hot and puffy. Remove from the pan and keep hot.
Add the chicken and fennel to the pan and fry for about 8 minutes, turning once until browned and cooked through. Meanwhile, make the dressing by mixing the yogurt with the mustard, honey and a little seasoning.
Slit the pitta breads and fill with the orange segments, watercress, chicken and fennel, then drizzle the dressing over and serve immediately.