Preheat oven to 180 degrees C. Press the sort crust into a baking dish then bake about 10 minutes.
In a saucepan saute butter and onion over medium high until tender. Stir in flour. Gradually add the milk, making a creamy sauce. Stir in the veggies and the tuna, simmering until frozen vegetables thaw.
Pour into the pastry base, spread top with mashed potatoes then bake in the preheated oven until golden brown on top, about 20 minutes.