In large skillet over medium heat, melt butter, add corned beef, cover, and cook 5 minutes.
Add coleslaw, cover, and cook 5 minutes. Stir to combine.
For dressing: In small bowl, mix dressing ingredients; set aside.
Top coleslaw and beef mixture with cheese. Cover until cheese is melted.
Divide reuben mixture into bowls, drizzle with dressing, and garnish with onions.