Add diced russet potatoes to a large pot and cover with cold water. Bring to a boil, reduce heat, and cook for 8-10 minutes. Drain and set aside.

Preheat the oven to 350°F (180°C). Grease a 2-quart baking dish with unsalted butter.

In a skillet, melt butter over medium heat. Add the diced onion and a pinch of salt, sauté until soft, about 5-7 minutes. Add green onions and garlic, sauté for another minute.

Stir in flour and cook for about 2 minutes. Gradually whisk in chicken broth and milk, cooking until thickened, about 3-5 minutes.

Remove from heat and stir in sour cream, salt, and pepper. Fold in the drained potatoes, ensuring they are coated in the sauce.

Spread the potato mixture into the greased baking dish. Top with cheddar cheese. Bake for 40-50 minutes until golden brown.

Let cool for about 10 minutes before serving.