Preheat oven to 350°F. Generously butter a large casserole dish and set aside. Line a baking sheet with foil or parchment paper.
Place the split croissants on the lined baking sheet and bake until crisp, about 5-10 minutes, cut side down. Remove and let cool. Coarsely crumble or chop the croissants into a large bowl and set aside.
Melt 2 tablespoons of the butter in a skillet over medium-high heat. Crumble and cook the sausage in the skillet until browned and cooked through. Transfer to the large bowl with the croissants. Saute the chopped onion, celery, and apple over medium heat until slightly soft, and season with salt and pepper to taste. Remove the from heat and toss with croissants. Add the herbs.
In a small bowl, whisk together eggs, 1 cup of the broth, and heavy cream. Pour over the croissants. Add additional broth to moisten to your liking. Transfer the stuffing mixture to the buttered casserole dish and top with remaining butter. Bake until crisp, about 30 minutes.