Preheat the oven to 250 degrees F.
Trim the crusts from the bread. Cut the bread into 1/2-inch cubes. Spread the cubes on a baking sheet and toast in the oven until dry but not yet browned, 20 to 30 minutes. Set aside.
Raise the oven temperature to 350 degrees F.
In a small saucepan or microwave-proof dish, melt 1 tablespoon of the butter. Brush a 2-quart (2-l) casserole or gratin dish with the melted butter and set aside.
In a large saute pan, add another 1 tablespoon of the butter and melt over high heat. Add the spinach and saute just until it has wilted. Drain the spinach, transfer to a bowl, and set aside.
To the same saute pan over medium heat, add the remaining butter. Add the onion and garlic and saute until translucent, 2 to 3 minutes. Add the button and shiitake mushrooms and saute, stirring frequently, for 5 minutes. Add the oysters, oyster juice, sausage, spinach, dried fruits, and salt and pepper. Cook for 3 minutes more, stirring frequently to combine the ingredients. Set aside.
In a large mixing bowl, lightly beat the eggs. Stir in the cream and milk until well blended. Add the parsley, sage, thyme, and rosemary. Add the reserved oyster mixture, bread cubes, and spinach and stir until thoroughly combined. Taste and adjust the seasoning, if necessary, with a little more salt and pepper.
Transfer the mixture to the buttered casserole dish. Bake until the dressing is heated through, puffy, and golden brown on top, about 45 minutes. Serve immediately.