Preheat oven to 450°F.
Add tuna, mustard, truffle oil and lemon juice to a bowl and stir, breaking up tuna into soft, small pieces. Add the olives, beans, parsley and green onion and mix until combined.
Butter the bread slices and place on a baking sheet, then bake until lightly toasted, about 5 minutes. Remove bread and preheat broiler.
Divide the tuna mix evenly on the toasted bread, spreading and using the back of a spoon to gently press the tuna down into a flat, even surface. Top with the tomato slices and cheese.
Place baking sheet in oven and broil until the cheese has melted and starts to brown, about 5-10 minutes. Serve warm.
Buon appetito!