Step 1Chop the onion into 6 to 8 pieces and puree them in a food processor. When completely smooth, transfer the puree to a fine strainer or double layers of cheesecloth and squeeze out all the juice; you should have about one-half cup juice. Discard the onion solids.
Step 2Cut the baguette crosswise into two 5-inch pieces. Split each piece in half lengthwise to make four long pieces and toast them.
Step 3Put the butter in a small skillet and melt it over medium heat. Add the onion juice and cheese and heat, stirring constantly, until the cheese is melted and the mixture is smooth, about 2 minutes. Stir in pepper to taste.
Step 4Let the cheese mixture cool for 2 to 3 minutes to firm up slightly. Ladle it onto the flat sides of the toasted baguette pieces, place on a foil-lined baking sheet with sides and broil until the tops bubble and are lightly browned, about 1 minute. Serve hot.