Heat the oven to 350°F. Peel and chop the potatoes into 1.5-2 inch chunks. Place in a pot and cover with cold water. Add a pinch of salt and bring to a boil over medium high heat and let simmer, uncovered, until the potatoes are fork tender, 13-15 minutes. Drain well.
Meanwhile, in a large saucepan, melt the butter over medium heat. When melted, stir in the flour, garlic powder, onion powder, and dry mustard, until smooth and incorporated.
Pour the milk into the pan in a thin stream while whisking, until smooth. Keep on medium heat, stirring occasionally, until the sauce bubbles and thickens, 3-5 minutes. Add half of the shredded cheddar and the sour cream and stir until melted and creamy.
Add in the drained potatoes, mixing until coated. Taste and season with salt and pepper.
Spread the cheesy potatoes into an oven safe dish and top with the remaining cheese. Bake for 20-30 minutes or until golden brown and bubbly. Remove, let cool slightly, and enjoy!