1.
If someone cooks up a dishonest scheme, they plan it. ( INFORMAL )
He must have cooked up his scheme on the spur of the moment.
PHRASAL VERB : V P n (not pron) , also V n P
2.
If someone cooks up an explanation or a story, they make it up. ( INFORMAL )
She’ll cook up a convincing explanation.
PHRASAL VERB : V P n (not pron) , also V n P