Heat a thin layer of olive oil in a large skillet over medium heat. Toast the bread slices on one side until lightly browned, 1 to 2 minutes, then flip them over. Top 2 slices with the Cheddar and Parmesan. Spread the other 2 slices with the labneh and sprinkle with a pinch of za'atar and sumac. Carefully sandwich each Cheddar/Parmesan slice with a labneh slice, cover with a lid and cook until the bottom is toasted, about 1 1/2 minutes. Flip, cover and cook until the other side is toasted and the Parmesan and Cheddar are melted, another 1 1/2 minutes. Transfer to a plate, cut diagonally and serve with tomato soup if desired.