Melt butter in a large pot over medium heat.
Add onions and carrots, cook until softened.
Stir in flour and cook for 2 minutes.
Gradually whisk in chicken stock and bring to a simmer.
Add chicken, cream, marmite, mustard, salt, pepper, peas, rosemary, sage, and thyme.
Simmer until thickened.
Season with salt and pepper to taste.
Preheat oven to 400°F (200°C).
Transfer filling to a baking dish.
Roll out the shortcrust pastry and place it over the filling.
Cut slits in the pastry for ventilation.
Bake for 30-35 minutes or until pastry is golden brown.