Preheat your oven to 350°F (175°C). Cut brioche into cubes, spread on a baking sheet, drizzle with olive oil, and bake for 12-15 minutes until golden.

Heat 1 tablespoon olive oil in a skillet. Add mushrooms and sauté for 4-5 minutes until tender and golden. Set aside.

In the same skillet, add another tablespoon of olive oil, then onions and celery. Sauté for 8 minutes until soft. Add garlic and cook for 1 minute, then stir in butter and herbs.

In a mixing bowl, whisk together eggs and vegetable stock. Fold in toasted brioche, sautéed vegetables, half of the mushrooms, and half of the fontina cheese.

Transfer the mixture to a baking dish, top with remaining mushrooms and fontina. Bake for 30-35 minutes until golden brown.

Remove from the oven, let cool for 5 minutes. Serve warm with black pepper and additional herbs.