In a large soup pot, heat olive oil over medium-high heat. Add green and yellow onions and cook until translucent (about 5 minutes). Stir in garlic first, then add potatoes and spinach.
Cook until spinach wilts.
Cook for 15 minutes or until the potatoes are tender. Season with salt and pepper to taste.
Pour soup into a blender or food processor, and blend until smooth.
Add milk and mix until creamy.
Return to pot, and heat through.