In a large pot, heat oil over medium heat. Add onion and garlic; cook until lightly browned, 4 to 7 minutes. Then add in spinach and let simmer until hot. Drain if more liquid has accumulated. Transfer to bowl and let sit until next step is completed.
In the same pot (emptied), warm milk over medium heat (make sure not to scorch milk), then whisk in cream cheese until melted, about 3 minutes. Add spinach, Worcestershire sauce, 1/2 cup mozzarella, and 1/4 cup chedder; stir to combine. Season with salt and pepper. Pour into a lightly oiled (or use non stick)1 1/2-quart shallow baking dish; sprinkle with remaining 1/2 cup mozzarella.
Bake until bubbly and golden brown, 20 to 25 minutes.