Preheat oven to 350 degrees and line a rimmed baking sheet with aluminum foil.
Melt butter in a nonstick skillet and add onion. Cook until softened.
Add Alouette cheese, Worcestershire sauce, and garlic powder and stir until cheese is melted. Stir in spinach and remove from heat.Season to taste with salt and pepper.
Cut bread in half lengthwise and place cut side up on prepared baking sheet. Spread spinach mixture on bread and sprinkle with mozzarella cheese.
Bake for 8 to 12 minutes or until golden around the edges.Slice and serve.