Preheat oven to 190 C / Gas 5. Poke about 10 holes into spaghetti squash; place on a baking tin.
Bake in the preheated oven until spaghetti squash is softened and partially cooked, about 25 minutes. Remove from oven; cool until easily handled. Reduce oven temperature to 180 C / Gas 4.
Cut spaghetti squash in half and remove seeds. Pull the squash flesh out with a fork.
Heat oil in a frying pan over medium-high heat. Add courgette, salt and pepper; cook and stir until softened, 3 to 5 minutes. Add pepper, onion and garlic; cook and stir until flavours combine, about 3 minutes. Stir in green beans; cook until bright green, about 2 minutes.
Stir chopped tomatoes and herb seasoning into the pan; simmer until tomatoes are warm, about 5 minutes. Add spaghetti squash and mix until combined. Cover with foil.
Bake in the preheated oven until spaghetti squash is tender, about 10 minutes.