Grease a 4- to 6-quart slow cooker pot with the butter.

In a blender, purée the spinach and tomato.

Place pumpkin into the slow cooker. Add the spinach mixture, the olive oil, paprika, garlic powder, and sea salt and stir well.

Cover and cook on LOW for 3 to 4 hours.

Sprinkle stew with the feta cheese and lemon juice just before serving.