Preheat oven to 375°F.
Combine onion and juice of lemon in a mixing bowl.
In the same bowl, stir together tomato paste, garlic, Herbes de Provence, pepper and EVOO. Add the tuna, olives, capers, parsley and mix to combine and break up tuna slightly.
Divide the tuna mixture between the two slices of bread, transfer to a baking sheet and top with the cheese. Toast in oven for about 10 minutes or until the cheese is melted and the tuna heated through.