Prep pasta by bringing a part of salted water to boil, then cook pasta until al dente about 5 to 6 minutes. We don’t want it all completely cooked through since we’re going to bake it off in the oven. Drain pasta and set aside in a large mixing bowl. Meanwhile, grease a 9x13 baking dish with a tablespoon of butter or olive oil.
In a 3 quart sauté pan, melt 3 tablespoons butter with 3 tablespoons olive oil. Add onions and sauté until translucent about five minutes. Season with a pinch of kosher salt and add the mushrooms. Sauté mushrooms until browned, and all the liquid has been absorbed, about seven minutes.
Add the garlic and the chopped herbs and stir to combine with mushrooms. Set aside in a medium bowl and return sauté pan to heat.
Melt 8 tablespoons of butter in the sauté pan. Add half a cup of all purpose flour to create the roux. Whisk flour to combine with butter, then pour milk in a slow steady stream. Stir constantly until the sauce has begun to thicken, about 10 minutes. The sauce will begin to coat the back of a spatula or wooden spoon.
Remove sauce from heat and add in shredded cheese. Stir until cheese is melted completely. Taste sauce and adjust seasoning as desired with a little pinch of kosher salt and ground black pepper.
Add sautéed mushrooms and stir everything together until combined. Pour the mushroom and cheese sauce over cooked pasta in a large prep bowl. Then pour mac and cheese into the prepared baking dish.
In a small bowl, stir together the olive oil with bread crumbs. Sprinkle bread crumbs over the top of the mac and cheese. Bake until golden brown and bubbly, about 30 to 40 minutes. Allow to cool 10 minutes before serving.