Preheat oven to 400°.
Pierce potatoes with a fork, and bake at 400° for 1 hour and 15 minutes or until tender. Cool. Cut each potato in half lengthwise; scoop out pulp, leaving a 1/4-inch-thick shell. Mash pulp with a potato masher. Combine milk and cream cheese in a large bowl, and stir with a whisk. Add potato pulp, 1 cup cheddar cheese, onion, salt, pepper, and spinach; stir well. Spoon potato mixture into shells; sprinkle each half with 2 tablespoons cheddar cheese. Place stuffed potatoes on a baking sheet; bake at 400° for 15 minutes or until thoroughly heated. Garnish with green onions, if desired.