POUR lemon juice over fish in shallow dish; set aside. Heat dressing in large skillet on medium heat. Add leeks and peppers; cook 8 min. or until tender. Stir in broth; bring to boil. Add fish; cover. Simmer on medium heat 5 min. or until fish flakes easily with fork. Remove fish from skillet; cover to keep warm. Reserve vegetables and cooking liquid for later use.
MIX hot water, butter, cornmeal and cheese until mixture forms soft dough. Divide into 12 portions; shape each into ball. Cook in large saucepan of boiling water 5 min. or until slightly firm; drain.
BLEND cooked vegetables and reserved liquid in blender until smooth. Return to skillet. Add spinach, olives and capers; stir. Bring to boil; cover. Simmer on medium heat 3 min. or until spinach is wilted, stirring occasionally. Add fish and dumplings; cook 5 min. or until heated through, stirring occasionally.