Line a 9 x 13-inch baking dish or pan with parchment paper and set aside. (If you don’t have parchment, you can spray the dish with non-stick cooking spray.)
Add both cereals, peanuts, pretzels and half of the M&M’s to a large mixing bowl. (I add the other half of the M&M’s last because they look prettier in the mix that way. You can skip this part if you want and just add all of the M&M’s in the beginning.)
Mix the ingredients together.
Add white chocolate and shortening to a microwave-safe measuring cup or bowl.
Melt in the microwave in 30-second increments until completely melted and smooth, stirring in between. (You can also use a double boiler to melt the chocolate, but I find the microwave easier.)
Add the melted white chocolate to the bowl with the cereals, peanuts, pretzels and M&M’s.
Mix melted white chocolate with the cereal ingredients using a large spoon or silicone spatula until everything looks evenly coated.
Spread the mixture evenly in the baking dish.
Let snack mix cool and harden in the refrigerator for 10-15 minutes. You can also let it sit at room temperature for 1 hour if you don’t want to put it in the refrigerator.
Carefully break up the snack mix with your hands.
Add the remaining M&M’s and toss.
Enjoy!