Preheat an oven to 180 degrees C. Lightly grease a 20x30cm baking dish.
Break spaghetti into 5cm pieces. Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the spaghetti and return to a boil. Cook the spaghetti uncovered, stirring occasionally, until it has cooked through but is still firm to the bite, about 12 minutes. Drain well in a colander set in the sink.
Combine onion and green capsicum with cooked spaghetti, 1/2 cup Cheddar cheese, turkey meat, mushroom soup, chicken stock, salt and pepper in a large bowl. Mix lightly and spread the mixture in the baking dish and top with 60g grated cheddar cheese.
Bake, uncovered, in oven for 45 minutes until the casserole is bubbling and cheese topping is melted and browned.