Bring a large pot of water to boil. Add the potatoes (whole with skin on) and cook until soft and fork tender, about 15-20 minutes. When the potatoes are done, transfer them to a bowl to cool off so they are easier to handle.
While the potatoes cool, begin on the sauce. Add the butter to a large pan over medium heat. While the butter is melting, dice the onion into small pieces then add to the pot. Stir while the onion cooks for about 4-5 minutes. Mince the garlic and add to the pot and cook for 1-2 minutes.
Add the flour and whisk to combine. Let it cook for 1-2 minutes to let the raw flour cook off. Slowly add the chicken broth and whisk to combine. While continuing to whisk, add the milk. The mixture should be thick but smooth.
Turn off the heat and add the salt, sour cream, and shredded cheese.
Preheat the oven to 350F. Carefully peel and shred the cooked potatoes using a box grater. Add the shredded potatoes to the pan and gently stir to combine.
Transfer the potato mixture to a 9x13" pan. Add the 1/4 cup of melted butter and honey bunches of oats (or cornflakes) to a bowl. Stir until the cereal is evenly coated in butter. Sprinkle the cereal on top of the potatoes and transfer to the oven.
Cook for 10 minutes until the cheese is melted and the cereal is golden brown. Enjoy!
