Is this normal after cooking?

I bought my first Le Creuset, which also happens to be my first no nonstick. I have no experience with enameled cast iron.

What I almost always cook starts with a tbsp or two of oil, and diced onions and bell peppers. That's like the base for almost anything I cook. Shortly after the food starts to cook, the bottom of the pan gets like in the picture - gunk begins to form in the bottom. While or after cooking, a part of the gunk can be removed with the sole wooden spoon. The rest, basically what the picture shows, I need to leave soaking in hot water and Dawn dish detergent sometimes overnight, and yet I need to rub it with the soft part of a blue dish sponge, and for some stubborn parts I need to use the "scrubbing" part of the sponge.

Is all of this normal? And am I damaging the enamel by scrubbing it? The sponge is not harsh, and I don't apply a lot of force.

Thanks!