1kg potatoes, scrubbed
3 cups milk (not fat-free), room temperature
1 large onion, thinly sliced
½ cup dry vermouth or white wine
8 slices bacon, cooked
½ cup grated Parmigiano-Reggiano
5 tbsps unsalted butter, softened
3 tbsps all-purpose flour
1 tbsp Dijon mustard
1 tbsp fresh chives, minced
1 tbsp fresh tarragon, minced
½ tsp salt
½ tsp freshly ground pepper, preferably white
1) Cook the potatoes until firm in a pot with salted water.
2) Drain and cool, then remove the skins.
3) Slice the potatoes into 0.5cm thick slices; set aside.
4) Melt 3 tablespoons of the butter in a large skillet over medium heat.
5) Whisk in the flour and cook for 20 seconds but do not let the flour brown.
6) Continue whisking as you add the milk in a slow, thin, steady stream; whisk until the mixture
thickens, about a minute.
7) Take the pan off the heat and whisk in the mustard, chives, tarragon, salt, and pepper. Set aside.
8) Melt 2 tablespoons of the butter in a medium skillet set over low heat.
9) Add the onion and cook until sweet, about 15 minutes, stirring frequently.
10) Reduce the heat if it begins to brown. Set aside once onion turns golden.
11) Position the rack in the center of the oven and preheat the oven to 180 °C.
12) Butter a 10-cup gratin dish or oval baking dish.
13) Spoon the sautÃ©ed onions into the bottom of the baking dish, spreading them out evenly across its
14) Place half the potato slices on top, overlapping them as necessary.
15) Sprinkle with 2 tablespoons of the vermouth, then spoon half the white bÃ©chamel over the top, spreading it evenly to the edges of the dish.
16) Lay the bacon slices evenly over the sauce, then top with the remaining potatoes, again overlapping them as necessary.
17) Sprinkle with the remaining 6 tablespoons vermouth, then spoon on the rest of the bÃ©chamel.
18) Top the dish with the grated Parmigiano-Reggiano.
19) Bake for 45 minutes, or until brown and bubbling.
20) Allow to stand for 5 minutes, then serve.