Preheat your oven to 375°F (190°C). Butter a 9x13 inch baking dish.

Cook pasta in salted boiling water until just al dente, about 1 minute shy of package time. Drain and set aside.

In a large skillet over medium heat, melt 2 tbsp of butter. Add the chopped onion and a pinch of salt; sauté until translucent, about 4-5 minutes. Add the mushrooms and cook until their liquid evaporates and they brown, about 6-8 minutes.

Stir in the garlic and cook for 30 seconds until fragrant.

Push veggies to one side, add remaining 2 tbsp butter to the skillet. Sprinkle the flour over the butter and whisk to form a roux. Cook 1-2 minutes to take off the raw flour taste.

Gradually whisk in chicken broth, scraping up browned bits. Add the milk/half-and-half, Dijon (if using), and thyme, and bring to a gentle simmer.

Cook until the sauce thickens enough to coat the back of a spoon, 3-5 minutes. Taste and season with salt and pepper. Stir in 3/4 cup of the Parmesan.

Remove from heat and fold in the cooked chicken and drained pasta, ensuring everything is evenly coated.

Transfer mixture to the prepared baking dish. Sprinkle remaining Parmesan evenly over the top, then scatter the panko-butter mixture for a crisp golden finish.

Bake uncovered for 20-25 minutes, until bubbling and the top is golden. Broil for an extra-crispy top for 1-2 minutes, watching closely.

Let rest for 5 minutes. Sprinkle with chopped parsley before serving.