The black stuff is probably burnt on pancake batter or similar which has polymerised. Gonna be tough to get off. OK, Le Creuset puts heat tolerant enamel on as the coating on their pans, so this might work. Enamel is a glass, which doesn’t react to strong alkali or acid but is fragile to impacts and scratching. Once cracked, it’s ruined as cookware as you really don’t want glass shards in your dinner.

First, fill the pan with water, add a squirt of dishwashing liquid and put the pan on a very gentle simmer for an hour or so. Watch it like a hawk so it doesn’t boil dry - top up with hot water as needed. Then let it cool a bit, tip out most of the water and scrub with a plastic scourer.

If that doesn’t work to lift the black bits, then its time for the heavy duty stuff:

1. Mix up a slurry of bicarb or baking soda with dishwashing liquid to make an alkali intense treatment - which degrades the polymerisation chemically - the consistency of wet snot is what you’re looking for. Slather this on the black bits and cover with a layer of cling film. Leave the pan in a warm spot for a day or so, then rinse off. Try scrubbing with a plastic scourer on the now weakened black bits. If some comes off, then repeat process once.

2. If there’s still some black bits remaining after that, then what remains is resistant to alkali. So try an acid intense treatment such as a vinegar/salt/flour slurry (with cling film) to further degrade the polymerised black bits. The paste recipe is 1/3 cup flour, 1/3 cup salt and enough vinegar to make a slurry. Put this ONLY on the black bits not the whole pan, and lay it on thicc. This too needs to sit on the black bits under cling film for a day or so and then scrubbed. Don’t soak the whole pan in straight vinegar as this will loosen the enamel from the underlying cast iron and cause it to flake off. Barkeepers Friend is an alternative to the paste recipie.

If none of these work either by themselves or together, then the pan is probably ruined. If the only stuff remaining is in a network of cracks, the pan IS ruined and should be discarded. Le Creuset probably won’t refund if the problem occurred as a result of leaving the pan on the heat after using it, as they advise prompt cleanup in their instructions.

I know, I know, it sounds like a whole lotta work. Le Creuset is expensive, and very good quality, but does need to be cared for properly and not abused.