Cook the Pasta

Bring a large pot of salted water to a boil.

Cook the spaghetti according to package instructions until al dente.

Drain the pasta and set aside, reserving a cup of pasta water for adjusting the sauce consistency later if needed.

Prepare the Roux

In a medium saucepan, melt the butter over medium heat.

Add the flour and whisk continuously to combine, cooking for about 2 minutes until you have a light golden roux.

Add Onions and Truffle Oil

To the roux, add the finely diced onions or shallots.

Cook for 2 minutes until they are soft and translucent.

Stir in the truffle oil and mix well.

Create the Sauce

Gradually whisk in the milk to avoid lumps. Keep whisking until the mixture is smooth.

Bring the mixture to a simmer, then reduce the heat to low.

Add the grated Parmigian cheese, black pepper, and salt to taste. Stir until the cheese has melted and the sauce thickens slightly.

Combine Pasta and Sauce

Add the cooked pasta to the sauce, tossing well to ensure each strand is evenly coated.

If the sauce is too thick, use the reserved pasta water to adjust to your desired consistency.

Serve

Serve the truffle pasta hot, with an optional extra drizzle of truffle oil or a sprinkle of grated cheese and black pepper on top for garnish.