Preheat the oven to 350 degrees F.
In a large heavy bottom pan, melt the butter over medium high heat. Once the butter is melted, add the onions, leeks and mushrooms and sauté for 10 minutes until the onions are translucent and then mushrooms are soft.
Add the sage and stir until fragrant, about 2 minutes.
Transfer the mixture to a big bowl and add the toasted cornbread cubes to the bowl.
Whisk together the egg, cream and stock in another small bowl and then pour it over the cornbread mixture. Toss everything together to combine and spoon the stuffing into a large baking dish.
Transfer the baking dish into the oven and bake for 25-30 minutes until the top of the cornbread is golden brown. Remove and serve immediately.