Preheat the oven to 350°F.

In a large skillet, heat 1 tablespoon of the olive oil over medium until shimmering. Add the chopped sage leaves, and saute until just crisp. Spoon them out of the pan (leaving the oil behind) and set aside on a plate.

Add the remaining oil, the sausage, and leeks, and cook until the sausage begins to brown up. Add the mushrooms, and cover the skillet with a lid or foil. Cook, stirring occasionally, until the mushrooms have given off their liquids, and the sausage is browned, and the leeks are soft. Season with salt and pepper, give another quick stir, then remove from the skillet and set aside to cool almost to room temperature.

In a very large mixing bowl, whisk together the eggs, dairy, and a big pinch each of salt and freshly ground black pepper. Fold in the bread cubes until completely coated, followed by the mostly-cooled mushroom mixture and the cheese.

Transfer to a 2 to 2 1/2 quart baking dish. Sprinkle the fried sage leaves over the top, cover the dish with foil, and bake until the edges are set (the center will still be a bit wobbly) for 25 to 30 minutes. Remove the foil and bake for 20 minutes more.