Roughly peel pounds of russet potatoes, leaving a little bit of skin on if desired. Slice the potatoes into planks, then strips and dice into cubes. Add the cubed potatoes into a pot of cold water, bring to a boil and cook for 8 minutes. Remove and drain the potatoes into a colander set into a sink. Let the potatoes cool.
In a 10-inch skillet add three tablespoons of butter. Over medium-low melt the butter and then add in the diced onions. Cook until just softened and add in the minced garlic. Cook for one minute.
Sprinkle in the flour and stir, cooking for 2-3 minutes. While stirring, pour in the 1/2 cup of broth and 1/2 cup of whole milk. Continue to stir until thick, about 5-8 minutes.
Remove the pan from the heat and stir in the cup of cold sour cream.
In a large bowl add in the cooled potatoes, onion and sour cream mixture, green onions, salt, pepper and grated Colby cheese. Stir to combine and pour in the prepared pan.
Bake in a 350 degree oven for 1 hour or until bubbly and lightly golden. Let cool for 20-25 minutes before serving.