COOK potatoes in half each of the dressing and rosemary in large nonstick skillet on medium-high heat 8 min., stirring occasionally. Reduce heat to medium-low.
ADD fish; sprinkle with lemon juice and remaining rosemary. Cover; cook 6 to 8 min. or until fish flakes easily with fork and potatoes are tender. Remove skin from fish; discard.
TOSS lettuce with cucumbers. Serve topped with the fish, potatoes amd remaining dressing.