Preheat oven to 200°C fan forced. Lightly grease an 8-cup capacity ovenproof dish. Cook the pasta in a large saucepan of boiling salted water, following packet directions, until al dente.
Meanwhile, melt the butter in a large saucepan over medium-high heat until foaming. Add the flour and cook, stirring constantly, for 1 minute. Remove pan from heat. Combine the milk and cream in a jug. Gradually whisk the cream mixture into the butter mixture until sauce is smooth.
Return the pan to medium-high heat. Cook, stirring constantly, for 5 minutes or until sauce comes to the boil. Stir in the mushrooms and onions. Cook for 2 minutes.
Drain the pasta and return to the hot saucepan. Add the mushroom mixture and cheese. Season with salt and pepper. Stir to combine. Spoon the pasta into the prepared dish. Top with breadcrumbs. Lightly spray the top with oil. Bake for 20 minutes or until golden and hot. Serve with salad or vegetables.