In a large bowl, combine the tuna, bell pepper, almost all of the scallions (leaving a few for garnish), chives, mayo, lemon juice and red pepper flakes and gently stir to combine.
Turn the oven to broil.
Arrange the sliced bread on a baking sheet and drizzle the bread with olive oil. Place under the broiler for 2 minutes until just starting to toast. Remove and rub the bread with the clove of garlic.
Divide the tuna mixture on top of each piece of bread and top with thin slices of the cheese. Transfer back into the oven and broil for 1-2 minutes until the cheese has melted. Remove from the oven, top with additional scallions, season with salt and pepper and serve.