Preheat the oven to 200'C / Fan 180C / Gas Mark 6. Grease a muffin tin with the oil, and set aside.
Using a biscuit cutter, cut a circle from each slice of bread. (See notes for what to with the leftover bread - don't waste it!) Gently press each slice into the muffin holes - don't worry too much if you get any breaks, you can tear some tiny bits of the leftover bread to patch them up a bit.
Bake the bread for 10 minutes until golden. Meanwhile make the filling by mixing the eggs and tuna in a measuring jug (season with salt and pepper to taste if you like).
Remove the bread from the oven and carefully fill each bread-cup to the top with the filling. Place back in the oven for another 10 minutes until golden on top. Remove and cool, then top with chopped chives if desired. Can be eaten hot or cold.