Preheat your oven to 400°F / 200°C. Lightly grease a 3-quart rectangular baking dish.
Cook the pasta according to package directions until al dente.
While the pasta cooks, prepare the frozen vegetables according to their package directions.
Drain both well and return the pasta to its warm pot.
For the cheese sauce, melt the butter in a medium saucepan over medium heat.
Whisk in the flour and cook for one minute, stirring constantly, to form a roux. Gradually pour in the milk, whisking continuously to prevent lumps.
Continue to cook and stir until the sauce thickens and begins to bubble.
Let it bubble gently for one more minute, then remove from the heat.
Stir in the ½ cup of Parmesan cheese, salt, and pepper until the cheese is fully melted and the sauce is smooth.
Pour about ½ cup of the finished cheese sauce over the drained pasta in the pot and toss gently to coat every strand.
Spread this saucy pasta evenly into the bottom of your prepared baking dish.
Arrange the cooked vegetables and drained mushrooms over the pasta.
Pour the remaining cheese sauce evenly over the top.
Sprinkle the final 2 tablespoons (30 ml / 1 fl oz) of Parmesan cheese over the entire dish.
Bake, uncovered, for about 15 minutes, or until the sauce is bubbly and the casserole is heated through.
Let it rest for a few minutes before serving.
