Preheat your oven to 350°F (175°C).
Cook the linguine according to package instructions until al dente. Drain and set aside.
In a large skillet, melt 2 tablespoons of butter over medium heat. Add onions and garlic; sauté until softened, about 3-4 minutes.
Add the sliced mushrooms to the skillet and cook for an additional 5 minutes until they are golden brown.
Sprinkle the flour over the mushroom mixture and stir to combine, cooking for another 2 minutes.
Gradually whisk in the chicken broth and heavy cream, stirring continuously until thickened.
Add the shredded chicken, peas, salt, pepper, and nutmeg to the sauce. Stir until well combined.
Fold in the cooked pasta until evenly coated with the sauce.
Transfer the mixture to a greased baking dish and top with breadcrumbs and grated Parmesan cheese.
Bake in the preheated oven for 25-30 minutes, until bubbly and golden on top.
Remove from oven and let it cool for 5-10 minutes before serving.