2 teaspoons olive oil, plus more for the baking dish
1 medium onion, finely diced
2 garlic cloves, minced
2 pounds spinach, cleaned, trimmed, and coarsely chopped, or 2 10-ounce packages frozen chopped spinach, thawed and thoroughly drained
1/4 cup stock or water
1/2 cup milk
6 ounces reduced-fat cream cheese
Worcestershire sauce, to taste
Hot sauce, to taste
1 cup grated mozzarella
Kosher salt and freshly ground pepper
Baguette slices, breadsticks, or crackers, for serving
Preheat oven to 425 degrees. In a Dutch oven or large pot, heat oil over medium. Add onion and garlic; cook until lightly browned and softened, 5 to 8 minutes.
Add spinach in two additions adding a portion of the stock or water with each and cover with a lid. Let 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 1/2 cup mozzarella; stir to combine. Season with salt and pepper to taste. 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.