This version of mac and cheese is inspired by the staff meals at Bistro d'à Côté, a restaurant where I interned in Paris. It's made with Gruyère cheese and béchamel sauce—and what I'm going for is a really creamy, cheesy interior and a crisp exterior.
For the béchamel: Heat the milk, onion, and bay leaf in a small saucepan over medium-low heat until steam wisps start to rise from the milk (this is scalding the milk, the stage just before boiling), then turn the heat to low. Meanwhile, bring water to a rapid boil in a large saucepan; salt the water. Add the macaroni, immediately stir with a wooden spoon to prevent sticking, and cook for 6 minutes, stirring occasionally.
As the macaroni cooks, prepare the roux: Melt the butter in a medium saucepan over medium heat; then whisk in the flour and cook, stirring, to remove the raw flavor of the flour. Once the mixture is bubbling, turn the heat down to low as it continues to cook, about 5 minutes. Add a pinch of cayenne. Slowly add the scalded milk, whisking vigorously after each addition until sauce is smooth. (Discard the onion and bay leaf.) The sauce will thicken as it heats, so be sure to whisk continuously to avoid burning. When all the milk is incorporated, bring to a simmer again before taking the saucepan off the heat. Drain the cooked pasta thoroughly.
Whisk the cream into the béchamel and season with salt and pepper. Place the macaroni in a large bowl and add 2 cups of the Gruyère. Pour the béchamel on top and stir well to combine. Season to taste with salt.
Assemble and bake the macaroni: Preheat oven to 400 degrees F. Pour macaroni into a buttered baking dish and top with the remaining Gruyère, then sprinkle on the panko and Parmesan. Bake until the top is lightly browned and cheese is bubbly, 20-30 minutes.
Remove macaroni from the oven and set aside. Chop chives and parsley and garnish before serving.