Thanks for the suggestions questseeker....
I change the coloring of the bread and slightly adjusted the shading on it.
I changed the seeds to lines instead of diamonds and I placed them more frequently around the top (i combined parts of your suggestion with another pixel art burger example I found that I liked).
The purple is supposed to be purple onions (burgers more often have white, but I thought purple looked better). I did some extra shading on those trying to give them depth and more obvious onion-ness.
Any followup thoughts or thoughts from anyone else? I think this does look a good deal better already although I'm not really sure how I should be shading the bun to make it look right.
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