Line a large baking sheet (or two smaller ones) with parchment paper or wax paper. Clear some counter space so you have room to spread the mix out to cool.
In a very large mixing bowl or clean roasting pan, add the Rice Chex, Corn Chex, pretzels, salted peanuts, M&M's, and mini marshmallows (if using). Gently toss with clean hands or a large spatula until evenly combined.
4 cups Rice Chex cereal, 3 cups Corn Chex cereal, 2 cups small pretzels, 1 1/2 cups salted peanuts, 1 cup plain M&M's or seasonal candy-coated chocolates, 1 cup mini marshmallows
Place the white chocolate chips and coconut oil or shortening in a microwave-safe bowl. Microwave on 50% power for 30 seconds, then stir. Continue microwaving in 20–30 second bursts at 50% power, stirring after each interval, until almost fully melted. Remove and stir until completely smooth to finish melting without scorching.
2 cups white chocolate chips, 2 tablespoons refined coconut oil or vegetable shortening
Stir the fine sea salt into the smooth, melted white chocolate until fully dissolved and evenly distributed.
1/2 teaspoon fine sea salt
Working fairly quickly, pour the melted white chocolate mixture evenly over the cereal mixture in the large bowl. Use a large spatula or spoon to gently fold and toss, scraping from the bottom, until all of the cereal, pretzels, nuts, and mix-ins are evenly coated. Keep folding; it will look like not enough chocolate at first, but it will spread.
While the white chocolate is still wet, scatter festive sprinkles over the top, if using. Give the mix one more gentle toss to distribute, taking care not to crush the cereal.
festive sprinkles
Transfer the coated mix to the prepared baking sheet(s), spreading it out into an even layer without large mounds so it sets evenly. Let cool at room temperature for 20–30 minutes, or until the white chocolate has fully hardened.
Once the mix is set and firm, break it apart into clusters or bite-sized pieces. Serve in a large bowl, jar, or individual cups, or portion into treat bags or tins for gifting.