Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish with olive oil or butter.

In a skillet, heat olive oil over medium heat. Sauté the diced onion and minced garlic until the onion is translucent.

Stir in chopped mushrooms, thyme, salt, and pepper; cook until mushrooms are golden brown and moisture is released.

Whisk together eggs, milk, heavy cream, and Dijon mustard in a large mixing bowl until smooth and creamy.

Add cubed bread to the egg mixture, followed by the mushroom mixture and shredded Gruyere cheese; fold together.

Pour the mixture into the prepared dish, spreading evenly; top with remaining Gruyere cheese.

Bake for 45 to 50 minutes until the top is golden brown and a knife comes out clean.

Let cool for 10 minutes before serving; garnish with freshly chopped parsley.