Preheat your oven to 400°F (200°C).
Peel the potatoes and cut them into thin matchstick-sized strips (julienned).
In a skillet, melt 1 tablespoon of butter over medium heat.
Add the thinly sliced onions and sauté until they are soft and translucent, about 10 minutes. Do not let them brown.
Grease a baking dish with a little butter.
Layer one-third of the julienned potatoes in the bottom of the dish.
Add half of the sautéed onions on top of the potatoes.
Sprinkle half of the chopped anchovies over the onions.
Repeat the layers, ending with a layer of potatoes on top.
In a bowl, mix the heavy cream and milk together.
Pour the cream and milk mixture over the layered potatoes, onions, and anchovies.
Sprinkle the breadcrumbs evenly over the top layer of potatoes.
Dot the top with the remaining butter.
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil and continue to bake for an additional 20-30 minutes, or until the potatoes are tender and the top is golden brown and crispy.
Let the dish cool slightly before serving.
Enjoy your meal!