Heat oven to 350ºF.
Boil pan of water and cook pasta according to package directions.
While pasta is cooking, melt the butter in a saucepan, add the onion and cook on low for 6-8 minutes or until the onions are soft.
Turn up heat to medium and stir in flour. Cook for 1-2 minutes.
Slowly add milk, stir with whisk until thickens. Add 2/3 of the cheese until melted. Season with salt and pepper.
Drain pasta and pour in to a baking dish. Pour over the white sauce, then add the turn, corn, peas and 1 tablespoon of the parsley. Mix together and sprinkle with the remaining cheese.
Bake for 15-20 minutes or until the cheese is golden brown. Top with remaining parsley before serving.
