Barkeepers friend (powdered metal and glass polish/cleaner, typically comes in a cannister) will get that off with a little bit of elbow grease.
Half the pans I’ve bought i got at a thrift store for like a buck because people thought they ruined them with a little bit of scorching., and I’ve gotten some nice stuff.
Unfortunately haven’t found that cheaply available in Finland. I know about it too. It’s the only thing I haven’t tried other than straight up sanding it
So the legend of bar keepers friend is that it was invented after someone boiled a bunch of rhubarb greens and noticed it cleaned the pan. I reckon any green high in oxalic acid (the main ingredient in BKF) should do similarly enough to the actual product to let you know if it might work.