Preheat oven to 350°F and grease an 8x11-inch baking dish with butter or nonstick spray. Set aside.

Place potatoes in a large pot of salted cold water and bring to a boil. Cook until firm but just tender, then drain and set aside to cool slightly.

In a large skillet over medium-low heat, melt the butter. Add onions and cook until softened, then stir in garlic and cook for 30 seconds more.

Stir in flour and cook 1-2 minutes. Slowly pour in chicken broth and half and half, stirring constantly. Continue cooking, stirring frequently, until thickened, about 5 minutes.

Remove from heat and stir in sour cream until smooth.

Mix together the cooked potatoes, sour cream mixture, green onions, and grated cheddar cheese in a large bowl. Season with salt and pepper to taste.

Transfer mixture to the prepared baking dish and bake until bubbly and golden, 40-50 minutes.

Let cool 10 minutes before serving. Enjoy!