Preheat the oven to 350°F. Grease a 1 ½-quart baking dish; set aside. Cook the spinach in the microwave according to the package instructions. Drain thoroughly by squeezing the spinach tightly in a dish towel to wring out the extra liquid.
In a medium saucepan over low heat, combine the soup, milk, cheese, sour cream, 2 tablespoons of the melted butter, salt, and pepper. Stir until just warmed through and combined. Add the spinach; mix well.
Transfer the spinach mixture to the prepared baking dish. In a small bowl, stir together the breadcrumbs or cracker crumbs with the remaining 1 tablespoon of melted butter. Sprinkle the buttered crumbs over the top of the casserole.
Bake the casserole, uncovered, for 30 minutes, or until it's heated through and golden brown on top.
