Preheat the oven to 400 degrees. Spread softened butter into a 9x13 glass baking dish.
Whisk together the milk, cream, flour, minced garlic, salt, and pepper. Set aside.
Slice the potatoes into sticks. Cut the sticks into minced cubes.
Spread half of the potatoes into the prepared baking dish. Pour on half of the cream mixture. Top with gruyere cheese. Add the remaining potatoes. Pour on the remaining half of the cream mixture.
Cover the baking dish with foil and bake for 30 minutes. Uncover and bake an additional 20 minutes. Test a potato for doneness, if fork doesn’t pierce easily continue baking, checking in 5-10 minute intervals until potatoes are tender.
Sprinkle with cheddar cheese. Bake an additional 3-5 minutes until cheese is melted. Sprinkle with chopped green onions.
Serve.