Instructions

1. 1
   Preheat oven to 450°F. Grease the baking dish.
2. 2
   Fill the pot with 4 quarts of water or enough to fully boil the pasta. Add 1 tablespoon salt and bring to a boil over high heat.
3. 3
   Melt 2 tbsp butter in the skillet over medium heat. Sauté the mushrooms and onion, stirring occasionally, until the liquid evaporates (around 7–8 minutes).
4. 4
   Add thyme with salt and pepper, and continue to cook until mushrooms are brown (another 8–10 minutes). Set the mixture aside in a bowl.
5. 5
   When the water is boiling, add the spaghetti. Cook until al dente (about 8–12 minutes). Taste periodically to ensure the pasta does not overcook prior to baking.
6. 6
   Return the skillet to the stove and melt 4 tablespoons butter over medium heat. To make a roux, add the flour and whisk until smooth with a light golden color (about 1–2 minutes).
7. 7
   While continually whisking, add a little chicken broth at a time to the roux. Fully incorporate until smooth before adding more broth.
8. 8
   Increase to medium-high and simmer until the sauce thickens (about 3–4 minutes).
9. 9
   Remove from the heat, then stir in the Parmesan cheese with the salt and pepper.
10. 10
    Add half of the lemon juice, then continue adding lemon juice to taste.
11. 11
    Combine the ingredients (chicken, spaghetti, mushrooms, peas, and sauce) together. Spread into the baking dish.
12. 12
    Sprinkle the bread crumbs evenly on top.
13. 13
    Bake until the dish is heated through and breadcrumbs are light brown, about 12–14 minutes.