Heat the olive oil in a large pan over a medium heat. Add the onion, carrots and leek. Season with salt and pepper, then cook for 5-6 minutes until slightly softened. Add the garlic, thyme and flour. Turn the heat to medium-high and cook for 2-3 minutes, stirring.
Add the cooked chicken along with the chicken stock. Bring to a simmer and cook for 10-12 minutes, or until the liquid is almost all evaporated. Stir in the cream, peas and sweetcorn and allow to bubble gently for another five minutes. Taste and add salt and/or pepper, if needed. Set aside.
Meanwhile, make the mash. Place the potatoes and turnips in a saucepan, cover with cold water and add a generous pinch of salt. Bring to the boil over a high heat, then reduce the heat to medium and simmer for 10 minutes or until very soft. Drain well, allowing to steam dry for 1-2 minutes, then mash well.
Combine the butter and milk in a small jug and microwave until the butter has just melted. Add to the mash and beat together until smooth. Season to taste.
Preheat the oven to 190˚C/170˚C fan/gas mark 5.
Transfer the chicken and vegetables to the Pyrex® Cook and Freeze Glass Rectangular Dish. Gently spread the mash over the top and brush with the beaten egg.
Bake for 30 minutes until the mash is golden on top and the filling is bubbling around the edges. Allow to rest for five minutes before serving straight to the table in the Cook and Freeze Glass Rectangular Dish.