Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium casserole dish.
Cook spinach according to package directions. Place potatoes in a pot with enough water to cover, and bring to a boil. Cook 15 minutes, or until tender but firm. Drain, cool slightly, and mash.
In a bowl, mix the spinach, mashed potatoes, butter, sour cream, onion, salt, and dill. Transfer to the prepared casserole dish. Top with Cheddar cheese.
Bake 20 minutes in the preheated oven, until bubbly and lightly brown.
