01 - Preheat the oven to 400°F (200°C) and lightly grease a 9 x 13 inch baking dish.
02 - Boil pasta in a large pot of salted water until just al dente. Drain thoroughly and set aside.
03 - In a large skillet over medium heat, melt 2 tablespoons butter. Add onion, carrots, and celery, sautéing for 5-6 minutes until vegetables are softened.
04 - Stir in garlic, thyme, parsley, black pepper, and salt. Sprinkle 3 tablespoons flour onto the vegetables, stirring and cooking for 1 minute.
05 - Gradually whisk in 1 2/3 cups broth and 1/2 cup milk. Simmer, stirring constantly, until the mixture thickens, about 3-4 minutes. Fold in peas and spinach if using. Remove from heat.
06 - Melt 2 tablespoons butter in a saucepan over medium heat. Whisk in 2 tablespoons flour and cook 1 minute. Gradually add 2 cups milk, whisking constantly until smooth and thickened, about 4-5 minutes.
07 - Remove saucepan from heat. Stir in grated cheddar, mozzarella, ground mustard, then adjust seasoning with salt and pepper.
08 - In a large mixing bowl, gently toss together the cooked pasta, pot pie filling, and cheese sauce until evenly blended. Transfer mixture to the prepared baking dish.
09 - Combine panko breadcrumbs, melted butter, and Parmesan in a bowl. Evenly sprinkle the topping over the pasta mixture.
10 - Bake in the center of the oven for 20-25 minutes or until the top is golden and the filling bubbles.
11 - Allow the dish to rest for 10 minutes before serving for ease of portioning.