Preheat the oven to 350 degrees F. Combine the pretzel pieces, corn cereal, bagel chips and peanuts in a large bowl.
Combine the butter, mustard, honey, Worcestershire, onion and garlic powders, 1 tablespoon of the mustard powder, red pepper flakes and salt in a small saucepan. Place over medium-low heat and cook, whisking, until the butter melts and the mixture just begins to bubble. Remove from the heat.
Pour the butter mixture over the pretzel mixture and toss until thoroughly coated. Spread out on a rimmed baking sheet and bake until lightly toasted and mostly dry, 20 to 22 minutes, stirring and tossing the mixture about halfway through the cooking time. Allow to cool for a few minutes on the baking sheet.
Rub the brown sugar and remaining 1 teaspoon mustard powder together with your fingers to make a fine powder. Transfer the toasted snack mix to a large bowl and toss with the mustard-brown sugar mixture. Let cool completely, and then store in an airtight container for up to 3 days.