In heavy skillet, brown sausage meat until ¾ cooked. Drain. Add onions, garlic and fennel. Cook until vegetables are tender. Deglaze with white wine. Reduce until almost dry. Add spinach. Cook until wilted.
In large bowl with bread cubes, add meat mix, cheese, 4 eggs. Mix well. Moisten with turkey stock, about 2 cups more if moister stuffing is desired. Bake at 350° for 45 minutes in foiled-covered baking dish. Moisten occasionally with stock. Remove and fluff. Season with salt, pepper and crushed red pepper flakes, if desired.