Lightly coat the bottom and sides of a medium baking sheet with the nonstick cooking spray or melted butter.
In a microwave-safe bowl, zap the marshmallows and nut butter, stirring after 1 minute, until melted, about 2 minutes.
Add the brown rice cereal, cranberries, pecans and cashews to the melted marshmallow mixture and stir gently to combine – be careful not to crush up the bits of cereal too much.
Either wet your hands or spray them lightly with the nonstick cooking spray and turn the cereal marshmallow mixture out onto the greased baking sheet. Press it into the pan using your slick hands so that it's evenly distributed. Set the pan aside and let the bars set up, about 30 minutes.
Once set, cut them into squares and serve.