In very large bowl or roasting pan, combine the cereals, pretzels, cashews, and M&Ms.
Melt the white chocolate in the microwave in thirty second increments, stirring as you go. (I use a quart-size Pyrex measuring cup for this.) Once it is completely smooth, pour over the cereal mixture. Gently toss until all of the cereal mixture is evenly coated. Carefully, turn mixture onto a large piece of parchment or wax paper to cool. Once cooled, store in an air-tight container.