Step 1

Preparation Arrange oxtails in single layer in heavy large pot. Add next 8 ingredients, then enough water to cover oxtails by 1/2 inch; bring to boil. Reduce heat to low, partially cover, and simmer until very tender, adding more water by 1/2 cupfuls as needed to keep oxtails covered, about 3 hours. Cool slightly. Chill uncovered until cold, then cover and keep chilled at least 1 day and up to 3 days.

Step 2

Spoon off and discard fat from sauce. Rewarm oxtails over low heat. Transfer oxtails to large plate. Boil sauce just until reduced enough to coat spoon thinly (do not reduce too much or sauce may become salty). Discard ginger slices and star anise.

Step 3

Meanwhile, cook yu choy in large pot of boiling salted water until just tender, about 4 minutes. Drain well.

Step 4

Divide yu choy among shallow bowls. Top with oxtails and sauce and sprinkle with green onions.