Preheat the oven to 400 degrees F (200 degrees C).
Combine Parmesan cheese, mayonnaise, onion, and reserved onion juice in a bowl; stir together until well blended.
Spread the mixture thickly on baguette slices and set on a large baking sheet.
Bake in the preheated oven until cheese is melted and bread is lightly toasted, about 10 minutes.
