Preheat oven to 400 degrees.
In a skillet preheated to medium heat, melt butter. Place onion into skillet and cook for a few minutes.
Add salt, pepper, sugar, Worcestershire sauce, and thyme. Stir and cook for another couple of minutes.
Add beef broth and cook until onions are golden brown - about 12 minutes. Make sure to stir occasionally to keep the onions from burning.
Spray a 5 X 8 baking dish with non-stick cooking spray. In a large bowl combine zucchini noodles and French onion mixture. Pour the French onion zoodle mixture into the dish and garnish top with Fontina cheese.
Place dish in the oven and bake for about 20 to 25 minutes or until golden brown. Remove from oven and cool slightly before serving.
Garnish with fresh thyme if desired. Also, there is a probability that water from the zucchini will appear. If so, just drain it off carefully with a spoon before serving. Enjoy!