Preheat oven to 425. In a Dutch oven (5 qt pot with a tight fitted lid) over medium heat,warm oil. Add onion and garlic;cook until lightly browned, 5 to 8 minutes.
Add spinach in two additions,letting the first batch wilt before adding the next; cook until completely wilted, 5 to 8 minutes. Transfer to a colander; drain,pressing to release all excess liquid.
In the same pot, warm milk over high heat. Whisk cream cheese until melted, about 3 minutes. Add spinach, Worcestershire sauce,hot sauce and 1/4 cup Mozzarella;stir to combine. Season with salt and pepper. Pour into a lightly oiled 1 1/2 qt shallow baking dish; sprinkle with remaining 1/2 cup mozzarella.
Bake until bubbly and golden brown, 20 to 25 minutes. Serve hot with accompaniments, as desired