- 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 batches, 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.