APlace the cereals, pretzels, bagel chips, other crackers, and nuts (if using) in a 6-quart (preferably oval-shaped) slow cooker. Using 2 wooden spoons, gently toss together, being careful not to crush the cereal
Whisk the butter, Worcestershire sauce, seasoned salt, garlic powder, and onion powder together in a small bowl or measuring cup. Pour half over the dry ingredients and toss to coat evenly. Pour in the remaining half and toss again.
Cover and cook on LOW for 3 hours, stirring every 30 minutes. When it is done, the party mix should look toasty and relatively dry.
Pour the party mix onto a work surface or baking sheets lined with parchment or paper towels. Spread into a single layer. Cool for 5 minutes before serving or storing.
Party mix can be stored in an airtight container for up to 1 week.