Preheat the oven to 375°F. Peel and dice the potatoes, finely chop the onion, and mince the garlic.
Add the potatoes to a large pot of water. Bring to a boil, cooking for 8-10 minutes until fork tender. Strain in a colander. Then, transfer the potatoes to a greased 9x13 casserole dish.
While the potatoes are boiling, melt the butter in a large saucepan or pot over medium heat.
Add in the onion and salt, and cook for a few minutes until softened. Add in the garlic and cook for a minute. Then, stir in the flour.
Slowly pour in the milk while whisking to incorporate.
Add in 2 cups of cheese and the sour cream. Stir until melted and incorporated.
Pour the cheese sauce over the potatoes. Gently combine until the potatoes are fully coated.
Top with the crushed cereal and the remaining 1 1/2 cups of cheddar cheese.