Combine the plain Greek non-fat yogurt, garlic, parsley, thyme, and oregano in a shallow bowl. Season with salt and freshly ground black pepper to taste.
Reserve 2 Tbsp. of the yogurt-herb mixture, cover and refrigerate.
In a bowl, coat the chicken on all sides with the remaining yogurt-herb mixture. Cover with plastic wrap and refrigerate 15 minutes.
Coat a nonstick grill pan with canola oil cooking spray and preheat over medium-high heat. Cook the chicken about 6 minutes per side, or until cooked through.
Spread 1 Tbsp. of the reserved yogurt-herb mixture on the bread. Place the lettuce and chicken on top.
Spread the remaining yogurt-herb mixture over the chicken and top with the cucumber and onion.