Step 1

Preheat the oven to 400°F (200°C). Butter a 10-by-14-inch gratin or casserole dish.

Step 2

Cook the macaroni according to the directions on the box. Drain, turn it into a large bowl, and toss it with the oil.

Step 3

Meanwhile, in a large skillet, fry the bacon over medium heat until brown but not crisp, about 10 minutes. Drain on paper towels and add to the cooked macaroni.

Step 4

While the macaroni and bacon cook, in a medium saucepan over medium heat, bring the milk to a very gentle boil, then reduce the heat to low to keep it warm.

Step 5

In another saucepan over medium heat, melt the butter. When the foam subsides, remove the pan from the heat. Whisk in the flour and continue to stir until a smooth, pale roux forms. Return the saucepan to medium heat and, still whisking steadily, begin ladling the hot milk into the roux, 1 cup at a time, waiting until one ladleful of milk is completely incorporated before adding the next.

Step 6

After all the milk has been added, continue to whisk until the sauce thickens and bubbles gently, about 2 minutes. Add the Parmesan, 2 cups of the Gruyère, and the salt and pepper, and stir until the cheese has completely melted.

Step 7

Pour the sauce over the macaroni and bacon, mix thoroughly, and pour into the buttered dish. Bake in the oven for 12 minutes. Sprinkle the remaining 1 cup of Gruyère over the gratin and continue to bake for 10 minutes more, until the top is golden and crunchy.

Step 8

Remove from the oven and serve immediately.