Prepare the Zucchini and Bread:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9×9-inch baking dish with butter or cooking spray.
2. Cook the Zucchini: Heat the olive oil in a large skillet over medium heat. Add the sliced zucchini and sauté for about 5-7 minutes until softened. Season with salt, black pepper, and fresh thyme. Remove from heat and set aside.
3. Prepare the Bread: While the zucchini is cooking, cube the bread into bite-sized pieces and set aside. You can use day-old bread for better texture in your strata.

Mix the Cheese Mixture:

4. Whisk the Eggs and Milk: In a large mixing bowl, whisk together the eggs, milk, and Dijon mustard (if using). Add the ricotta cheese, mozzarella, cheddar, Parmesan, oregano, and minced garlic. Stir until the mixture is smooth and well-combined.

Assemble the Strata:

5. Layer the Bread: Start by placing half of the cubed bread in the bottom of the prepared baking dish, pressing down gently to create an even layer.
6. Add the Zucchini: Evenly distribute the sautéed zucchini over the bread layer.
7. Add the Cheese Mixture: Pour half of the cheese and egg mixture over the zucchini and bread.
8. Layer Again: Add the remaining bread cubes on top of the zucchini and pour the rest of the egg and cheese mixture over it.
9. Press and Smooth: Press down on the strata gently to ensure that the bread is well-soaked with the egg mixture.

Bake the Strata:

10. Bake: Place the baking dish in the oven and bake for 45-50 minutes, or until the top is golden brown and the center is set. You can check by inserting a toothpick or knife into the center of the strata; it should come out clean or with just a few moist crumbs.
11. Cool: Allow the strata to cool for 10 minutes before slicing and serving.

Garnish and Serve:

12. Garnish: Sprinkle with fresh parsley for a burst of color and freshness before serving. The strata can be served warm or at room temperature.