Preheat oven to 200°C. Cook the pasta in a saucepan of boiling water following packet directions or until almost al dente. Drain well.
Place the white sauce in a large bowl. Stir in the mustard. Add the pasta, corned beef, spring onion and spinach and fold until well combined. Season. Spoon the mixture into a large baking dish or ovenproof frying pan.
Combine cheddar and breadcrumbs in a small bowl and sprinkle over the pasta mixture. Cover with foil and bake for 30 mins. Remove foil and bake for a further 15 mins or until golden and the pasta bake is heated through. Divide among serving bowls.