The snack mix can be made one week ahead and stored at room temperature.
Preheat oven to 325 degrees. In a large bowl, combine cereal, pretzels, almonds, peanuts, and toffee.
Melt butter in a small saucepan over medium-high heat, stirring until golden brown, about 5 minutes. Remove from heat and stir in thyme. (Butter will splatter.) Stir in brown sugar, mustard, and salt. Add to cereal mixture and toss until evenly coated. Spread in an even layer on a rimmed baking sheet.
Bake, stirring halfway through, until nuts are toasted and toffee begins to melt, about 15 minutes. Let cool and discard thyme.