The duty to mitigate may be on you as the employee, but the burden of proof is on your employer as to whether or not you have taken reasonable measures to find new employment. This is why we tell employees to keep a log of their search, and why we help our employer clients to make regular inquiries as to how that search is going.
If the employer alleges that you have failed to mitigate, they have a high burden of proof to show that you have not lived up to your responsibilities. However, courts have ruled on cases where the employee has barely made an effort because they believed that their partner’s wages provided sufficient household income, or they only began looking over a year after being let go. In these cases courts have reduced the notice period the employee is owed, ruling that while an employee is allowed to make their own choices in these instances, employers should not be required to foot the bill for those choices.