1. Using a sharp knife, cut the tuna into strips, then cut each strip in half lengthways, then lay all the strips horizontally across your chopping board. Now slice along the tuna as thin as you can into small dice.
2. Place the tuna into a medium sized bowl and set aside.
3. Using the back of a knife, crush the anchovy fillet into a paste and put into a small bowl, add the rest of the ingredients and mix well, now add to the tuna and season with salt and pepper.
4. Toast the sourdough until slightly charred, rub with garlic and spread with sour cream.
5. Divide the tuna mix between the 4 toasts, top with the caviar and serve with a twist of pepper, rocket leaves and olive oil.