Step 1

Preheat the oven to 375°F with a rack in the center position.Bring a large pot of salted water to a boil over high heat

Step 2

Add the pasta and cook until 1 minute shy of al dente according to the package instructions

Step 3

Drain.Meanwhile, heat the olive oil in a large ovenproof braiser or Dutch oven (see Note) over medium heat

Step 4

Once the oil is glistening, add the onion and mushroom stems and cook until tender, about 4 minutes

Step 5

Transfer to a plate and set aside

Step 6

Melt 4 tablespoons of the butter in the same braiser over medium heat

Step 7

Once the butter begins to bubble, add the flour, stirring, until a thick paste forms, 1 to 2 minutes

Step 8

Gradually whisk in the milk until the sauce has thickened enough to coat the back of a spoon, 6-7 minutes

Step 9

Whisk in the crème fraîche, dijon, garlic powder, nutmeg, thyme, salt, and pepper until combined

Step 10

Stir in the cheddar, Gruyère, and parmesan until melted, then stir in the pasta until coated

Step 11

Top the macaroni and cheese with the mushroom caps, upside down

Step 12

Cut the remaining 2 tablespoons of butter into small pieces and place inside the mushroom caps.Bake in the oven until the mushrooms are tender, cheese is golden and the sauce is bubbling, 30 minutes

Step 13

Sprinkle with chives before serving, if using.