Method
Preheat the oven to 375°F and grease a 9x9-inch baking dish with 1 tablespoon of softened butter.
In a large pot of boiling water, cook the diced potatoes and butternut squash for 8–10 minutes, until tender but not mushy. Drain well.
In a medium bowl, mix the heavy cream, garlic powder, thyme, salt, and black pepper until combined.
Spread the cooked potatoes and squash evenly in the greased baking dish. Pour the cream mixture over the top and sprinkle with the chopped onion.
Top evenly with shredded cheddar cheese and dot with the remaining tablespoon of softened butter.
Bake uncovered for 25–30 minutes, until the cheese is bubbly and golden brown. Let cool for 5 minutes before serving.