01 - Preheat oven to 400°F (200°C). Grease a 9 x 13 inch (23 x 33 cm) baking dish.
02 - Boil the pasta in a large pot of salted water until just al dente, following package directions. Drain well and set aside.
03 - In a large skillet over medium heat, melt 2 tablespoons butter. Add onion, carrots, and celery; sauté for 5–6 minutes until softened. Stir in garlic, thyme, parsley, black pepper, and salt. Sprinkle 3 tablespoons flour over vegetables and cook, stirring, for 1 minute.
04 - Gradually whisk in broth and 1/2 cup milk. Bring to a gentle simmer, stirring constantly, until thickened, approximately 3–4 minutes. Stir in frozen peas and baby spinach (if using). Remove from heat.
05 - In a medium saucepan over medium heat, melt 2 tablespoons butter. Whisk in 2 tablespoons flour and cook for 1 minute. Gradually add 2 cups milk, whisking constantly, until sauce is smooth and thickened, about 4–5 minutes.
06 - Remove pan from heat. Stir in grated cheddar, mozzarella, and ground mustard until cheese is fully melted. Season with salt and pepper to taste.
07 - In a large mixing bowl, gently combine cooked pasta, pot pie filling, and cheese sauce until evenly mixed. Transfer mixture to prepared baking dish.
08 - In a small bowl, mix panko breadcrumbs, melted butter, and Parmesan cheese. Evenly sprinkle topping over the pasta mixture.
09 - Bake uncovered for 20–25 minutes, until the topping is golden brown and the filling is bubbling.
10 - Remove from oven and let stand for 10 minutes before serving.