Slice the onions thinly. Try to keep them even. This helps them cook properly. Onions give sweetness to the pie.

Heat butter and oil in a pan. Add onions and cook slowly. Stir often. Cook for 10–15 minutes until soft and golden.

Sprinkle flour over the onions. Stir well. Cook for one minute. This helps thicken the filling later.

Slowly pour in milk while stirring. Add salt, pepper and mustard. Keep stirring until the mixture becomes thick and smooth.

Remove the pan from heat. Add grated cheddar cheese. Stir until it melts into the sauce. The filling should look creamy.

Preheat oven to 180°C. Grease your pie dish lightly. This prevents sticking and helps easy removal.

Roll out pastry and line the pie dish. Press gently into corners. Trim extra edges for a neat shape.

Pour the cheese and onion mixture into the pastry. Spread it evenly. Make sure it fills the dish well.

Place another pastry sheet on top. Seal edges by pressing with fingers. Cut a small hole in the center for steam.

Brush the top with beaten egg. Bake for 35–40 minutes. The top should be golden and crisp.