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In order of intensity: Simmer water and baking soda for about 20 minutes, then scrub with an enamel-safe brush (nylon brush). You can also make a water+baking soda paste and gently massage that into the residue. Dawn Powerwash (the brand-name stuff, not a DIY recipe). Spray generously, cover with plastic wrap, and let it sit overnight if necessary. Le Creuset’s enameled cast iron cleaner. If all else fails, use Easy Off with the yellow cap (sodium hydroxide, AKA lye). Do NOT do this indoors, go outside and make sure there is a crosswind or use a fan. Wear gloves, a mask, and eye protection, and protect your work area with cardboard. Put the skillet in a garbage bag, spray the affected area until there’s a good thick coating, then twist the garbage bag closed and let it sit for a couple of hours until the gunk turns to slime or is dissolved enough for your regular ECI-safe scrubber to get the job done. Always wear gloves when touching/checking on the skillet and make sure to wash it thoroughly when done to make sure no raw lye is left behind.
Take a look around to see whether they might have le creuset cleaner on hand, like this: https://a.co/d/03AHxtRm