Set a large pasta pot of heavily salted water over high heat to come to a boil.
Melt the butter in a large pot over medium heat. Add the flour to make a roux. Stir the roux well so that it cooks evenly and turns a golden blond. Whisk in the cream and milk a little at a time, so there are no lumps. Add the garlic, whole sprigs of thyme, and bay leaves.
Simmer over medium heat, stirring ocassionally to prevent the bottom of the pot from scorching until it thickens, about 10 minutes. Season with salt and pepper.
Cook the pasta in the boiling water according to the package directions. Drain.
Mix the sauce with the cooked pasta. Immediately add the cheese and mix well so the heat and motion melt and stretch the cheese through the pasta. Transfer to a baking dish (I spray mine with cooking spray to prevent sticking) and top with the breadcrumbs. Cover with foil.
Bake approximately 40 minutes until warmed through and the cheese has melted. Remove the foil and switch the oven over to broil for the final 10 minutes.
Remove from the oven and drizzle with olive oil and sprinkle with the chives.