I can't compete with her when it comes to cooking.
Literal
Cooking [in-では] I [topic-は] her [to-に] can't-match.
A direct, stripped-down version of the 'can't match someone' pattern. では (in the area of) frames the domain plainly without conditional framing. に (without は) marks the comparison target matter-of-factly. かなわない is the negative of かなう (to rival/measure up).