Preheat oven to 325. Rinse corned beef under cold water. In a Dutch oven, heat oil over medium high. Add corned beef, fat side down, cook, turning once, until browned on both sides, 5-7 minutes. Remove to a cutting board and cut ½ inch slices.
In same pot, cook onions over medium heat, stirring until golden, 3-5 minutes. Add broth, beer, garlic and pepper.
Layer corned beef slices and potato slices in onion broth mixture.
Cover and bake 2 ½ hours. Add greens to pot. Cover and bake 15 minutes longer.