I've heard that you can put some water in it and boil it for a while to help loosen the burnt on stuff.
Good luck!

DD just burnt a pan the other night. She scoured it (before she told me) but it was still looking pretty bad. I put water and a couple of tablespoons of baking soda in the pan, let it boil for about 10 minutes, running a knife around the edges as it boiled. Then I rinsed and ran it through the dishwasher and it came out good as new, no scrubbing. Can't hurt to try--good luck.

My first thought was oven cleaner. However, before did that on an expensive pan such as yours probably is I'd go here and ask their advice:

http://www.lecreuset.com/usa/content.php?name=Contact_Us

Boil with baking soda....and soak for a while....then use a Magic sponge....and what doesn't come off after all this is best considered as "seasoning"...won't hurt a thing....and with food in the pot you won't even see it.
Linda C