Sprinkle the lamb with salt on all sides, add to a large Ziplock bag or small baking dish. Pour beer over the lamb, seal bag (or cover bowl). Refrigerate for 6 hours and up to 24.
Remove from beer, rinse and pat dry. Allow to sit at room temperate for 30 minutes. Sprinkle with pepper.
Preheat oven to 350.
Heat olive oil in a cast iron skillet until hot but not smoking. Add the sage, cook until slightly crispy and dry looking, about 2 minutes. Remove from oil, allow to drain on paper towels.
Add the lamb, searing on all sides until browned, about 3 minutes.
Transfer the pan to oven, allowing lamb to cook until it reaches an internal temperate of 120 to 125 (use a meat thermometer), about 10 minutes.
Remove from oven and allow to rest for 5 to 10 minutes.
In a pot over medium heat, melt the butter. Add milk, bring to a simmer.
Whisk in the polenta, once the pot starts to look dry and the milk is mostly obsorbed, add the beer. Simmer until polenta is tender and thickened, whisking occasionally, about 18 minutes.
Stir in salt, pepper, garlic powder, and onion powder.
Plate the polenta, sprinkle with goat cheese.
Cut the lamb between the bones, plate over polenta, sprinkle with crispy sage.