This rich, creamy dip can be assembled up to 3 days ahead, then covered and refrigerated. Or use it as a side dish!
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