Head a large pan over medium heat and add the oil. Add onion and garlic and cook until translucent, about 6 to 8 minutes.
Add spinach and cook until completely wilted, 5 to 8 minutes. Transfer to a colander and drain, pressing the spinach down with the back of a wooden spoon to release all excess liquid.
In the same pot, warm the milk over high heat. Whisk in cream cheese until smooth and melted, about 3 minutes. Add spinach, Worcestershire sauce, red pepper flakes, and mozzarella and stir to combine. Season with salt and pepper. Simmer over medium low heat until ready to serve. Transfer to serving dish and serve with chips.