Preheat the oven to 400°. Butter an 8-inch square baking dish. Whisk the heavy cream, milk, flour, salt and a few grinds of pepper in a large bowl until the flour is incorporated. Add the potatoes, scallions, chives and thyme and stir. Pour into the baking dish and press the potatoes down until they are mostly submerged in the cream. Cover the dish with foil and bake for 40 minutes. Remove the foil and continue baking until the potatoes are tender and the sauce has reduced and clings to the potatoes, about 20 minutes more. Sprinkle on the gruyère and continue baking until the cheese is browned and bubbly, 5 to 7 more minutes. Top with more chives.