Preheat oven to 325°F. Coat a 9-by-13-inch baking pan with cooking spray.

Cook sausage in a large nonstick skillet over medium-high heat, stirring and breaking up with a wooden spoon, until browned, about 10 minutes.

Add apple, mushroom, onion, and celery; cover, reduce heat and cook, stirring occasionally, until tender, about 10 minutes.

Transfer the mixture to a large bowl. Season with salt and pepper. Add cornbread, parsley, and sage.

Bring water to a simmer in a small saucepan and add the butter and bouillon.

Pour 1 cup over the stuffing mixture and toss gently (the cornbread will break into smaller pieces). Add as much of the remaining broth as needed, 1/2 cup at a time, until the stuffing feels moist but not wet. Spoon the stuffing into the prepared pan and cover with foil.

Bake the stuffing until thoroughly heated, about 25 minutes. Serve warm.