Cut pork into eight slices; flatten to 1/2-in. thickness. In a large resealable plastic bag, combine the onion, lemon juice, parsley, garlic, thyme and pepper; add pork. Seal bag and turn to coat; refrigerate for 4 hours or overnight. In a small bowl, combine the cucumber sauce ingredients. Cover and refrigerate until serving.
Drain pork and discard marinade.
Place on a broiler pan coated with cooking spray. Broil 4 in. from the heat for 6-8 minutes on each side or until juices run clear.