Preheat the oven to 190°C.
Heat the olive oil in a large, heavy-based saucepan over a medium heat. Sweat the onion and garlic for 5-6 minutes, stirring occasionally until softened. Add the pepper, aubergine, courgette and seasoning and continue to cook over a medium heat for 5-6 minutes until the vegetables are soft.
Remove from the heat and adjust the seasoning as necessary. Fold in the tuna chunks carefully, making sure you don't break them up too much.
Meanwhile, roll the pastry out on a lightly floured surface to ½cm thickness. Line an 8 inch ceramic pie plate with the pastry, cutting off the overhanging pastry (of which there should be a lot). Gather the excess pastry into a ball and roll out to a 9 inch circle of pastry, roughly ½cm in thickness and reserve to one side. Fill with the tuna and vegetable filling. Cover with the reserved pastry and seal well at the edges. Brush with the beaten egg to brown. Make a small slit in the top of the pie to allow any steam to escape as it is baking.
Bake for 45-55 minutes until the pastry is golden brown all over and the filling is hot. Remove from the oven and allow to cool for a few minutes before slicing and serving.