Preheat oven to 350°F. Spread the cubed bread on large rimmed baking sheet and transfer to the oven. Bake until bread is dry, about 10 minutes. Remove from the oven and set aside. 

Generously grease an 8x8 or similar baking dish. (To fill a 9x13 baking dish, double this recipe).

Meanwhile, melt butter in heavy large skillet over medium-high heat. Add mushrooms and shallots and sear until tender, about 5 minutes. Add the herbs, garlic, salt, pepper and red pepper flakes to the vegetable mixture. Sauté for 2 minutes more.

Add white wine and let the wine condense for a minute. Remove the pan from heat and add about 1/3 cup mascarpone to pan (you should have about 2 tablespoons mascarpone remaining) and let it melt into the herby mushroom mixture. Combine the bread cubes with the mushroom mascarpone mixture.

Whisk the eggs and 1 1/4 cups cool broth in a large measuring cup. Add egg mixture to stuffing mixture and toss to coat.

Transfer the mixture to prepared baking dish. Scatter the remaining mascarpone across the top in pea-sized chunks, then sprinkle the with parmesan. Cover the casserole dish with aluminum foil. Bake 25 minutes. Remove foil. Bake until golden, about 20 additional minutes. For a crispy top, broil for the final 2 minutes.

Serve with crispy sage leaves. (To make, fry fresh sage leaves in a shallow pan of olive oil for a few seconds until crisp. Remove to a paper towel and sprinkle with salt).