Simmer potatoes, water, and 1/4 teaspoon salt in 12-inch skillet. Cover; cook over medium-low 8 to 10 minutes until just tender.
Uncover; evaporate water 2 minutes. Add 1 tablespoon oil; cook over medium-high 6 to 8 minutes, turning occasionally, until several faces browned. Transfer to bowl.
Pat steak dry; add remaining salt and pepper. Add remaining oil; sear in 2 batches 3 to 4 minutes per batch, browning several faces, until largest pieces 145°F. Transfer to clean plate.
Reduce to low; melt butter. Stir garlic and rosemary 30 to 45 seconds without browning garlic; add Worcestershire.
Return steak, juices, and potatoes; gently toss over low 1 minute. Remove from heat; rest 3 minutes and divide among 4 plates.