Preheat oven to 180 degrees C. Grease a casserole dish.
Bring a large pot of salted water to a boil, add noodles and cook until al dente; drain.
In a large mixing bowl combine noodles, tuna, celery and spring onion. Stir in sour cream, mustard and mayonnaise. Season with salt and thyme. Spoon 1/2 of the noodle mixture into the prepared casserole dish. Arrange a layer of zucchini over the mixture. Top with the remaining noodles followed by a layer of zucchini. Top the entire casserole with cheese.
Bake in preheated oven for 30 minutes or until hot and bubbly. Sprinkle the casserole with tomatoes before serving.