Drain the tuna cans and divide the fish into small pieces.
Boil the egg noodles according to package directions. Drain through a colander and rinse with hot water.
In a large bowl, toss the noodles with the tuna, celery, and green onions.
Add the sour cream, mustard, mayonnaise, dried thyme, and salt. Mix until combined.
Butter a baking dish. Spread half of the tuna noodle mixture in the bottom. Layer half the zucchini circles on top, then add the remaining tuna noodle mixture and top with the remaining zucchini circles.
Sprinkle the shredded cheese evenly over the casserole.
Bake at 175°C (350°F) for 30 minutes until heated through and the cheese is melted.