Prep the bread: Cut day-old bread into ¾–1" cubes. If using fresh bread, spread cubes on a baking sheet and dry in a 250°F (120°C) oven for 15–25 minutes until just crisp; cool completely.
Make aromatics: In a large skillet over medium heat, melt 4 tablespoons butter with the olive oil. Add diced onion and celery with a pinch of salt; sweat gently 8–10 minutes until translucent and tender (don’t brown heavily). Add minced garlic and cook 30–60 seconds until fragrant. Remove from heat.
Add herbs & seasonings: Stir in sage, thyme, parsley, rosemary (if using), black pepper and nutmeg; cook 30 seconds to bloom the flavors.
Prepare optional add-ins: If using sausage, brown it thoroughly and drain. Sauté mushrooms until their liquid evaporates. Lightly cook apples or crisp bacon. Let add-ins cool slightly.
Combine bread & aromatics: In a large bowl or roasting pan, combine bread cubes, the aromatic mixture, and any optional add-ins. Toss gently to distribute.
Mix liquids: Whisk together 2 cups stock, beaten eggs, and the milk (or cream if using). Slowly pour liquid over the bread mixture while tossing gently. Stop when the texture resembles a wrung-out sponge; add up to ½ cup more stock only if needed.
Rest: Let the mixture sit 15–30 minutes at room temperature so the bread absorbs liquid evenly. (If making ahead, cover and refrigerate now — see Notes.)
Bake (casserole method — recommended): Preheat oven to 350°F (175°C). Butter a 9x13" or similar casserole dish. Transfer the stuffing, dot the top with the remaining butter, and cover tightly with foil. Bake covered 30 minutes, then remove foil and bake another 15–20 minutes until the top is golden and crisp.
(Optional) Stuff the bird: If stuffing the turkey (note safety below), do so immediately before roasting and make sure the center of the stuffing reaches 165°F (74°C).
Finish & serve: Let the stuffing rest 5–10 minutes after baking. Serve warm, garnished with a few fresh sage leaves or chopped parsley.