Melt the butter in a medium pot over medium heat. Add the onion and garlic, season with salt and pepper and cook, stirring occasionally, until softened, about 6 minutes. Add the cream cheese, Worcestershire, dry mustard, paprika and cayenne, and stir until melted. Add the spinach and crabmeat, and stir until warm and bubbling. Stir in the cheeses and season with salt and pepper.
Transfer the dip to a small baking dish and top with the panko. Place the baking dish on a baking sheet and bake until bubbling and golden, about 15 minutes.