It's like trail mix dessertified.
Mix It Up: Line a 9-by-13-inch pan with foil and spray with cooking spray. In a large pot, melt the butter over high heat. Add the bag of marshmallows and 1/2 tsp. salt and, using a silicone spatula, stir until smooth. Remove from heat. Add the cereal, oats, raisins, walnuts, fennel seeds, cinnamon, and nutmeg. and mix until evenly coated. Mix in the remaining 2 cups marshmallows, stirring just until incorporated but not melted.
Top It Off: Transfer the cereal to the prepared pan. Spray a clean spatula with cooking spray; pat the cereal into an even layer. Let cool for 2 hours. In a small bowl, stir confectioners’ sugar and water and, using a fork, drizzle over the treats. Let stand until set, about 1 hour. Cut into squares.