Preheat oven to 350°F.
Defrost the frozen chopped spinach completely. Place the spinach in a colander and press it with paper towels until as much moisture as possible is removed.
Ensure the cream cheese is at room temperature and soft.
In a medium bowl, combine the room-temperature cream cheese and the sour cream and stir until smooth.
Add the 1 (1-ounce) package ranch dressing mix, the drained spinach, and 1/2 cup of the shredded cheddar cheese to the bowl. Mix until evenly combined.
Transfer the mixture to a shallow baking dish and spread into an even layer.
Bake in the preheated 350°F oven for 20 minutes.
Remove the dish from the oven, sprinkle the remaining 1/2 cup shredded cheddar over the top, and return to the oven for 5 minutes, or until the cheese is melted.
Remove from the oven, let cool a few minutes, and serve (for example, with tortilla chips).