Drizzle the cut-side of the bread with olive oil, then put on a sheet tray and place in the oven to toast, 3 to 5 minutes. Evenly spread the butter between the 2 pieces.
In a non-reactive bowl, combine the blue cheese, cream cheese, Worcestershire and vinegar until well mixed. Add the heavy cream slowly and whip with a rubber spatula or whisk until smooth and mousselike. Reserve until ready to use.
Slice each tomato into four thick slices, then sprinkle with salt and pepper.
Spread a thick layer of the blue cheese mousse on the bottom half of the prepared loaf. Add the sliced tomatoes and liberally spread the shaved onion. Top with the top piece of bread and slice into servings.
Serve with plenty of napkins.
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