Combine 25ml of the tepid water with the yeast and leave in a small bowl for 5 minutes to dissolve. Combine the flour and salt in a large mixing bowl, making a well in the centre. Add the yeast and water mixture and the olive oil, mixing thoroughly until combined. Gradually add the remaining tepid water to the mixture, mixing until you have a dough.
Turn the dough out onto a lightly floured surface and knead for 8-10 minutes until you have a smooth, elastic dough. Form the dough into a ball and rest in a lightly oiled mixing bowl. Cover the bowl with a damp tea towel and leave to rise in a warm place for 1 hour or until doubled in size.
Once risen, knock the dough back. Transfer back to the mixing bowl and cover again with a damp tea towel, allowing it to rise in a warm place until doubled in size; usually 20-30 minutes.
Knock the dough back once more once rise, then knead for a few minutes.
Preheat the oven to 200°C.
Divide the dough into two even balls and roll out into rounds. Arrange the tuna, sundried tomato and mozzarella on top of one of the rounds of dough. Brush the rim with the beaten egg, then place the other round of dough sitting neatly on top. Secure the edges of the dough using your fingertips, then transfer to a lightly oiled baking tray.
Dimple the top of the focaccia using your fingertips, then bake for 20-25 minutes until risen and golden on top. Remove and cut into portions before serving.