Preheat the oven to 275ºF (135ºC) and line 2 sheet pans with parchment paper.
Add the Chex cereals, mixed nuts, pretzels, and the mix-ins of your choice into a very large bowl. (Note: if your mixins are chocolate and can melt, hold off on adding those in until the very end).
In a separate microwave-safe bowl, add the stick of butter. Heat in 30-second intervals until completely melted. Alternatively, you can melt it over low-medium heat in a saucepan on the stovetop.
Add the Worcestershire sauce, seasoned salt, garlic powder, and onion powder to the melted butter and stir well to combine.
Pour half of the melted butter mixture over the cereal and mixins, and toss well to coat. Add the remaining half of the butter mixture, and toss well until everything is evenly coated.
Spread the prepared Chex mix evenly over each sheet (don’t include any meltable mix-ins like chocolate or M&M’s - add those in after the mix has cooled).
Bake in the preheated oven for 25-35 minutes, stirring every 10-15 minutes, until lightly browned and crisp.
Remove from the oven and allow to cool completely on the baking sheets.
After the snack mix has cooled to room temperature you can add any mix-ins that weren’t added earlier (like M&M’s or chocolate), and serve.
Transfer to a bowl with a tight lid or an airtight container.
Notes
Storage: Chex Mix should last 1-2 weeks in an airtight container at room temperature.Slow cooker method: Cook on "low" for 3 hours, stirring every 60 minutes to prevent burning. No bake method: After adding the seasoned butter mixture to the snack mix, cool, then store for up to 1 week at room temperature. Gifting: Add the desired amount of Chex mix into cellophane bags or mason jars. Finish with a pretty ribbon or bow and a custom gift tag.