Preheat oven to 350°F.
Butter a casserole dish and add in hashbrowns. Set aside.
In a medium saucepan over medium heat, melt butter. Add onions and saute until onions are translucent.
Add in flour to start to form your roux, once flour is combined add in heavy cream and continue to whisk away any clumps until sauce as begun to slightly thicken.
Add in about a half a cup of cheddar cheese, salt and pepper and whisk until combined. Add more cheese if needed and set aside.
In the casserole dish or a large bowl mix together the cheese sauce, sour cream, additional shredded cheese if you like it extra cheesy and additional salt and pepper if you’d like.
The mixture should be thick and creamy. Spread evenly in a casserole dish (if you mixed everything in a bow)l and top with shredded cheddar cheese.
Bake in the oven for about 25-30 minutes OR until nice and bubbly and cheesy.
Remove from oven and cool for a couple of minutes. Enjoy!