Add olive oil onions & optional arugula to a skillet. Cook onions until translucent and golden. Add in spinach and cook just until wilted.
Add to a blender with 1/4 cup water and blend until smooth. set aside.
In a soup pot, add butter and flour. Cook, stirring constantly for about two minutes. Add milk and whisk until smooth. Cook, stirring occasionally for 5 minutes.
Stir in pureed spinach. Cook another 5 minutes and you're done! Enjoy!