Line two large rimmed baking sheets with parchment or wax paper.
Add the Chex cereal, pretzels, and optional nuts to a very large bowl. Stir gently to combine.
Chop the white chocolate into small pieces and place it in a microwave-safe bowl.
Microwave at 50% power in 20–30 second bursts, stirring well after each one. Stop when most of it has melted, then stir until smooth. Keep all tools completely dry so the chocolate doesn't seize.
Pour the melted chocolate over the cereal mix. Fold gently from the bottom up with a rubber spatula until evenly coated. Work quickly before the chocolate hardens.
Add the Christmas M&Ms and sprinkles. Fold once more, without overmixing, so the colors stay bright.
Spread the mix in an even layer on the prepared baking sheets. Sprinkle with flaky salt if you like.
Let it set at room temperature for 45–90 minutes, or refrigerate for 20–30 minutes to speed things up.
Break the mix into clusters with your hands. Serve right away or store.