Preheat the oven to 350°F. Spread the bread cubes on a sheet pan and bake for 20 minutes, until dried out and lightly browned. Set aside.
8 cups bread cubes from a rustic country loaf, 1/2" dice
Meanwhile, heat the oil and butter in a large 12-inch skillet over medium heat. Add the bacon and cook for 5 minutes, until it begins to brown.
2 tbsps. olive oil, 1 tbsp. butter, 2 oz. (2 slices) thick-cut bacon, diced
Stir in the leeks and cook over medium heat for 10 minutes, until tender. Add the mushrooms, sage, white wine, 1 tablespoon of kosher salt, and 1½ teaspoons of pepper. Cook for 12 minutes, stirring occasionally, until most of the liquid evaporates.
4 cups sliced leeks white and green parts (about 4-6 leeks), 1.5 lbs. cremini mushrooms, sliced, 1 tbsp. chopped fresh sage, 1/4 cup dry white wine, salt and pepper
Remove from the heat and stir in the parsley. Pour the mixture over the bread cubes on the baking tray and let cool slightly.
1/3 cup minced fresh-flat leaf parsley
In a large mixing bowl, whisk together the eggs, cream, chicken stock, and half of the Gruyere cheese. Add the bread cubes and mushroom mixture, stirring well to combine. Set aside at room temperature for 30 minutes to allow the bread to absorb the liquid.
6 large eggs, 1-1/2 cups heavy cream, 1 cup chicken stock, 8 oz. grated Gruyere cheese
Stir well and pour the mixture into a 3-quart gratin dish (13 x 9 inches). Sprinkle with the remaining cheese and bake for 45 to 50 minutes, until the top is browned and the custard is set. Serve hot.