Preheat your oven to 250°F (120°C). This low temperature is key for slowly toasting the mix without burning it.
In a large bowl, combine the Corn Chex, Rice Chex, and Wheat Chex cereals. Add the mixed nuts, pretzels, and bagel chips. Gently mix these dry ingredients, being careful not to crush the cereal.
In a small microwave-safe bowl, melt the butter. This should take about 30 seconds in the microwave. Once melted, stir in the Worcestershire sauce, seasoned salt, garlic powder, and onion powder until well combined.
Pour the butter mixture over the cereal mixture in the large bowl. Use a large spoon or your hands (if the mixture is cool enough) to toss and coat all the ingredients evenly with the seasoning.
Spread the mixture in an even layer on a large baking sheet. If you don't have a large enough sheet, you can use two smaller ones.
Bake in the preheated oven for 1 hour, stirring every 15 minutes. This frequent stirring is crucial for even toasting and to prevent burning.
After an hour, remove the baking sheet from the oven and let the Chex Mix cool completely. As it cools, it will continue to crisp up.
Once cooled, store your homemade Chex Mix in an airtight container. It should stay fresh for up to two weeks, though it rarely lasts that long!