It adds emphasis and makes the sentence sound folksier and more friendly. The sentence is perfectly grammatical and correct with the "up" removed, but then the sentence sounds cold and unfriendly.
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AnonymousWhat does 'up' do in this sentence?It modifies the meaning of 'cook' slightly. 'up' pulls the meaning toward 'concoct' and adds aspects such as 'improvising', and 'without delay' and 'informally'.