What is the burden of proving a plaintiff’s failure to mitigate in a wrongful dismissal action?

In Wrongful Dismissal, op. cit., the authors describe the onus on the defendant as being the proof of “non-mitigation” by the plaintiff. At p. 4-270.1, the burden on the defendant is stated: The onus rests upon the defendant to assert the plaintiff’s failure to mitigate (Michaels v. Red Deer College (1975), 1975 CanLII 15 (SCC), [1976] 2 S.C.R.324, [1975] 5 W.W.R. 575, 5 N.R. 99, 75 C.L.L.C. 14,280, 57 D.L.R. (3d) 386), 1975 CarswellAtla 57, 1975 CarswellAtla 142). The defendant must prove not only the plaintiff’s failure to mitigate damages but also must show that had the plaintiff taken reasonable steps to mitigate, he or she could, in fact, have likely obtained alternative employment.