Mitigation depends on the market. For regulators, that means a going-forward view.
If regulators allow recovery of some stranded costs, they should at least ensure that utilities operate their generating plants in a manner consistent with the actions taken by other owners of similar resources that participate in competitive markets for bulk power.
A priori estimates of stranded costs are almost certain to be wrong. Therefore, regulators should adjust recovery to reflect actual events (em in particular market prices for electricity.