Heat a large, deep skillet with a lid over high heat and add 1 tablespoon of oil. When hot, reduce heat to medium-low and add 10 dumplings, crimped side up. Cook, uncovered, until lightly browned on the bottom, about 4 minutes. (Do not turn them over.) Pour 1 cup boiling water into the skillet. Cover and steam-cook dumplings over medium heat for 8–10 minutes, until tops are translucent and water is mostly evaporated. Transfer to a plate and keep hot. Repeat process with remaining dumplings, oil, and water.