Preheat the oven to 375.
If you haven’t made your gluten-free cornbread in advance, proceed with this recipe. The cornbread can be made up to a week in advance and stored in the fridge.
Once cooled, cut the cornbread into ½-inch cubes. Add to a large mixing bowl.
Meanwhile, melt the butter in a large ovenproof skillet or saucepan over medium-low heat. Add the sage and cook until just fragrant, about 1 minutes. Transfer the butter to a small bowl or liquid 1-cup measure.
Return the same skillet to a medium flame and sauté the fennel and shallots in the dregs of the butter, adding more butter or oil if needed, until soft and translucent, about 5 minutes. Add the mushrooms, garlic and 1 teaspoon salt and continue sautéing until the mushrooms release some of their moisture and reduce in size, about 3 minutes. Turn the heat up to high and add the wine, scraping up any bits from the bottom of the pan. Reduce by half, then remove from the heat. Add the veggies to the mixing bowl with the cornbread and allow to cool. Reserve the skillet (no need to clean it).
Once room temperature, stir in the eggs and stock until well incorporated. Transfer the mixture to a casserole dish or return it to the same skillet or saucepan you were already using, if ovenproof. Drizzle the gluten-free cornbread stuffing evenly with the sage butter. You can also use a pastry brush to ensure every crouton gets covered.
Bake in the oven for 20 minutes, until cooked through and turning golden brown on top. If your stuffing isn’t browning enough, turn up the oven to 425F for 5 minutes. The stuffing can be made a day in advance up until this point. To serve, return to the oven at 425 degrees for 5 to 10 minutes, or until a crust has formed on top.