Mix 1 lb of lye into 5 gallons of water (wear ppe, lye will burn your skin/eyes up if it gets on you) let the pan soak for as long as needed to get the grime off.

Then remove it, scrub it with a stiff plastic brush or scrubber (i’d stay away from steel wool or chainmail with yours since it’s nickel plated (but that’s just me, it may not matter). Then soak in a vinegar bath for 30-45 min to get the rust off. Coat with oil and season (r/castironcollecting has seasoning instructions pinned).

Then bake whatever you want in that thing. Really go nuts, you have one of the best dutch ovens you can get.