Pre-heat your oven to 200°C.
Peel and cut potato into big chunks. Boil in salted water until tender all the way through.
In a large pot or heavy based pan with sides, add a good glug olive oil followed by the mushrooms and cook on medium heat. Saute until they get some colour and soften slightly.
Follow this with the garlic and onion, saute until softened.
Throw in the rest of the vegetables and herbs, turn up to medium/high heat, and cook until they get a little colour.
Add lentils, passata, tomato paste, beef stock cube. Add between ½ - 1 cups of water and let the mix simmer (covered) for 15-20minutes.
All the vegetables should be cooked by this point. If they aren't or the mix seems dry, add a little more water and cook for longer. You should have tender veggies and a filling that's saucy and not wet, before it goes into the oven.
Time to make the mashed potatoes. If you're adding frozen peas, throw them in the water for 1min then strain the lot. Add olive oil spread and mash that baby up. If it needs a little loosening to become smoother, you can add a little water or nut milk. Season if needed.
In a 20x20x4cm baking dish, layer in the filling and spread the mash on top. You want peaks of potato which will get nice and crispy. Drizzle with a little olive oil and bake until golden on top.