Heat 2 tablespoons oil in a large straight-sided skillet over medium heat. Add onions, leeks, celery, and garlic. Cook until onions are translucent and tender, about 20 minutes.
Add mushrooms, and cook until soft, about 5 minutes. Transfer mixture to a plate, and let stand until cool, about 5 minutes. (Onion-mushroom mixture can be refrigerated overnight.)
If making sausage stuffing: Return skillet to medium heat, and add remaining 2 tablespoons oil. Stir in sausage, and cook, breaking into small pieces, until browned, about 9 minutes. Remove from skillet using a slotted spoon, and transfer to paper-towel-lined plates. Let drain. (Sausage can be refrigerated overnight.)
Combine onion-mushroom mixture, bread, apricots, sage, and thyme in a large bowl. Just before baking, pour stock and melted butter over top of mixture. Season with salt and pepper. Mix to combine.
To make sausage stuffing: Transfer 5 cups stuffing to another large bowl, and mix in sausage.
Let stuffings stand for 10 minutes, allowing bread to soak up liquid.
To cook mushroom stuffing: Preheat oven to 375 degrees. Place stuffing in a 9-by-13-inch baking dish, and bake until top is crisp and stuffing is heated through, 50 to 55 minutes. Let stand for 10 minutes before serving.
To cook sausage stuffing, follow directions for stuffing spice-rubbed roast turkey (recipe above), or preheat oven to 375, transfer stuffing to a 9-by-13-inch baking dish, and bake until top is crisp and stuffing is heated through, 45 to 50 minutes. Serve warm.
To dry the cubed bread, leave it out overnight or spread it on a baking sheet and heat it in a 200-degree oven. The onion-mushroom and sausage mixtures can be made one day in advance and mixed with the bread and liquids just before cooking.