Preheat the oven to 400 degrees F (200 degrees C).
Heat the 2 tablespoons of olive oil in a medium saucepan over medium heat and add the chopped leek, carrots, green beans and celery and sauté for about 2 minutes until slightly softened.
Stir in the kidney beans, lentils, dried herbs and chili flakes.
Pour over the stock and stir in the cornstarch slurry, followed by the tamari, Worcestershire and rosemary sprig.
Lower the heat to medium low. Bring to a gentle simmer for around 10 minutes. Allow to cool slightly, about 5 minutes. Discard the rosemary sprig.
For the mashed potatoes: Peel and cut the potatoes into even pieces, so they cook at the same time. Put them in a large saucepan and cover with water. Add 1 pinch of salt. Bring to a boil, then reduce the heat to a simmer. Cook until they are fork tender, around 10 to 12 minutes, depending on the size.
Drain in a colander, then return back to the saucepan. Mash really well.
Drizzle in the extra-virgin olive oil, mustard, black pepper and 1 pinch of sea salt. Stir in the unsweetened plant-based milk and use a fork to mix and fluff it up, getting rid of any lumps at the same time. Finally, stir in the parsley.
Spoon the bean mixture into a large baking dish, approximately 12-by-8 inches (2 quarts). Spoon the mash on top and use a fork to even it out.
Bake in the oven for 20 to 25 minutes, until golden brown and crisp on top and bubbling.