Place corned beef along with the contents of spice packet into a pot. Cover with water until 2 inches above the beef. Bring to a boil, reduce heat, and simmer for 2 hours 15 minutes. Maintain the water level by occasionally adding more water.

Remove corned beef to a bowl and cool for at least 30 minutes. Reserve 1 cup of the cooking liquid. Trim fat and cut corned beef into bite-sized pieces.

Heat oil in a pan over medium-high heat, add mushrooms with salt, and cook until browned (about 3 minutes). Add onions and continue cooking until soft and translucent (about 3 minutes). Add butter and stir until melted.

Make space in the center of the pan, add garlic, and sauté for 30 seconds. Add flour and cook for 1 minute. Pour in white wine and reserved cooking liquid, stirring to combine. Bring to a boil and reduce the sauce by half (3-5 minutes).

Add beef broth and boil until sauce thickens (6-7 minutes). Stir in creme fraiche and continue reducing until desired thickness.

Stir in sliced corned beef and cook at medium-low heat until heated through (2-4 minutes). Sprinkle with fresh dill.