Make the fruit topping first: Place sliced banana, blueberries, strawberries, raspberries, sugar, water and cornflour into a medium sized saucepan and place over a low heat.
Bring to a boil, stirring frequently, and then cook for 2-4 minutes until slightly thickened and the raspberries and strawberries have cooked down into a sauce and the blueberries and bananas are soft, but still have a little bite.
Leave to cool whilst you make the French Toast: Place the eggs, milk, sugar and vanilla into a shallow bowl and whisk together with a fork until combined.
Place 1tbsp butter into a large frying pan/skillet and place over a medium heat. Once the butter is melted and sizzling, dunk a slice of bread into the egg mixture for a couple of seconds and then flip and dunk the other side.
Place in the frying pan and fry each side of the bread until golden, 1-2 minutes per side. You may have to cook in batches if your pan is not big enough! (Mine wasn't - hence the need for more than 1tbsp butter!)
Transfer toast to a plate while you make the rest of the French toast. (Unless you have a huge pan!) Before you fry each batch, place a bit more butter into the pan and wait for it to sizzle before adding the bread.
Once all the bread is fried. Place 1 or 2 slices of bread on a plate and top with a spoonful of Greek yoghurt in the centre of the slice of toast, if using. Then add a spoonful of the banana berry mixture and serve immediately.
Note: If your bread is a little fresher than you would like, leave it out on a tray for a couple of hours before you make the French Toast and it will dry out!