Preheat the oven to 350 degrees while you line a baking dish with parchment paper.
Melt 2 tablespoons of butter in a large skillet over medium-high heat then saute the onion, mushrooms, and garlic for about 8 minutes. Remove from the heat and stir in the cooked macaroni.
Melt 2 tablespoons of butter in another medium sized saucepan over low heat. Whisk in the flour, cooking for about 2 minutes then stir in the soy milk. Bring to a boil over medium-high heat whisking constantly to avoid clumping. Reduce heat to low and simmer for 6-7 minutes then whisk in the salt, pepper, and mustard.
Pour sauce over mushroom mixture in the skillet; stir to coat. Spoon into the prepared baking dish then top with Panko breadcrumbs and drizzle with olive oil.
Cover with aluminum foil and bake for 15 minutes; uncover and bake for another 10 minutes or until gently browned on top. Enjoy!