Preheat oven to 425 degrees. In a Dutch oven or large pot, heat oil over medium. 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 in 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-quart 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.
The recipe can be assembled through step two up to three days ahead, then covered and refrigerated; it can also be served as a side dish. Two 10-ounce packages of frozen spinach can be substituted for the fresh. Thaw according to box instructions, drain well, and chop.