Place flours, oats and salt together in a food processor and process to a fine meal. Add oil and process until combined. Add trim milk gradually and process until pastry comes together. Be careful not to make it too wet. Allow to rest for around 30 minutes before using.
Heat oil in a heavy bottomed pan. Add onion and cook over medium heat for a couple of minutes; add mince and brown. Add carrot, peas, stock and tomato sauce or relish and cook, covered, for around 15 minutes.
Mix flour and water together to a paste and add to meat; stir until sauce has thickened. Season to taste with salt and pepper and cool before using.
Divide pastry into 2 portions, one slightly bigger than the second. Roll out the bigger portion to the shape of the pie dish, plus about 4-5cm larger all around. Line bottom and sides of a 22cm pie dish (or just use pastry as topping, as per photograph). Add cooled filling and then top with second pastry round (22cm diameter). Press sides together to seal. Brush top with a little milk and then pierce with a fork or knife in many places to allow steam to escape.
Bake at 190°C for around 20 minutes and then at 180°C for a further 10-15 minutes. Serve hot with salad.