To start, brown the butter. In an 8-inch skillet, melt the butter with the thyme and rosemary over medium heat. Once melted, continue to cook until the foaming slows down and the butter becomes a deep golden color and smells nutty, 6-8 minutes. Remove from heat and cool 5 minutes.
While the butter is cooling, in a large bowl, combine the Corn Chex, Oat Squares, Wheat Chex, Parmesan fish crackers, sesame sticks, pretzels, and mixed nuts. Stir to combine and set aside.
Once the butter has cooled, remove the thyme and rosemary, and set aside. Add the onion and garlic powder, seasoning salt, and Worcestershire sauce. Whisk to combine.
Drizzle a few tablespoons of prepared butter mixture over the party mix and stir well. Continue to drizzle and stir in small amounts to ensure the cereal mixture is evenly coated.
Pour onto two parchment-lined baking sheets. Spread out to an even layer and bake in the preheated oven for 1 hour, stirring and switching racks every 15 minutes.
Once baked, remove from the oven. Cool 20 minutes and serve or store in an airtight container in a cool place for up to one month.