Preheat the oven to 350 degrees F (175 degrees C). Spray a 2-quart casserole dish with cooking spray.

Fill a large pot with lightly salted water and bring to a rapid boil. Cook egg noodles at a boil until tender yet firm to the bite, 7 to 9 minutes. Drain and transfer to a large mixing bowl.

Add tuna, celery, and green onions to the noodles. Stir in mayonnaise, sour cream, mustard, thyme, and salt until well combined.

Spoon 1/2 of the noodle mixture into the prepared casserole dish, then layer 1/2 of the zucchini slices over top. Repeat layers once more, then sprinkle Monterey Jack cheese over the entire casserole.

Bake in the preheated oven until hot and bubbly, about 30 minutes.

Remove from the oven and sprinkle with chopped tomato before serving.