Filter by slug from URL (advanced)
Why don't you use your current entry as reference?

{% set nutritions = craft.entries.section('nutrition').relatedTo({
    field: 'restaurant',
    targetElement: entry
}).all() %}


Since your current entry is related to the entry you want to fetch this would be the besser solution. However that URL structure is not really optimal to beginn with. There are many cases it will break as soon as someone edits your entries, but filtering by slug alone is never a good idea in such a way.