Preheat the oven to 350°F and grease a 9x13 inch baking dish. Spread the thawed hash browns on a clean kitchen towel, roll it up, and squeeze out as much moisture as you can so the casserole bakes up creamy instead of watery.
Melt the butter in a medium saucepan over medium heat. Add the onion and cook for 4-5 minutes, until soft and translucent, then whisk in the flour and cook for 1 minute.
Slowly whisk in the chicken broth, followed by the oat milk, and simmer for 3-4 minutes until thick and smooth.Remove the pan from the heat and stir in the sour cream, garlic powder, smoked paprika, salt, and pepper.
In a large bowl, toss the hash browns with the creamy sauce and 1 1/2 cups of the cheddar until evenly coated. Spread the mixture into the prepared dish and sprinkle the remaining cheese on top.
Toss the crushed cornflakes with the melted butter and scatter them evenly over the casserole.
Bake for 50-55 minutes, until bubbling around the edges and golden on top. Let it rest for 10 minutes before serving so it sets and slices neatly.