Preheat the oven to 170 °C.
Place a large frying pan or large wide-open saucepan over medium heat and add the olive oil. Once hot, add the diced onion and fry for around 3-4 minutes or until just starting to brown.
1 tablespoon olive oil, 1 onion
Add the celery and carrot and fry for another 3-4 minutes until the mixture is softened and very fragrant. Add the vegan mince and break up with your spatula. Fry for another 3-4 minutes or until the mince has started to brown lightly.
1 stick celery, 1 carrot, 300 g vegan mince
In a measuring jug, whisk together the tomato puree, ketchup, boiling water, vegetable stock cube, red wine, marmite, vegan Worcestershire sauce, soy sauce and thyme leaves. Add to the pan and stir. Bring to a simmer then reduce the temperature slightly.
1 tablespoon tomato puree, 1 tablespoon ketchup, 400 ml water, 1 vegetable stock cube, 100 ml red wine, 1 teaspoon marmite, 1.5 teaspoon vegan Worcestershire sauce, 4 sprigs thyme, 1 tablespoon soy sauce
Leave the sauce to simmer for 15 minutes, stirring occasionally to ensure it's not catching. Add the frozen peas then simmer for a further 2 minutes.
100 g frozen peas
In a small bowl, whisk together the corn starch and 1 tablespoon cold water. Add to the sauce and stir then leave to simmer and thicken for a further 2 minutes.
1 teaspoon cornstarch
Season the sauce to taste with salt and pepper then transfer to a 23cm/9 inch casserole dish. Top the filling with the prepared mashed potatoes a little at a time, working from the edges of the dish into the centre.
1 pinch ground black pepper, 1 teaspoon flaky sea salt
Place the pie in the oven to bake for 25-30 minutes before removing, topping with the vegan cheese and placing under the grill or broiler for a further 5 minutes.
110 g vegan cheese
Ideally, allow the pie to rest for 10-15 minutes before serving. This will help it keep its shape when scooped.