Heat a few glugs of olive oil a large saucepan, and cook the carrots and onions until they start to soften (maybe 3-5 minutes).
Add the mushrooms, herbs and mustard powder. Cook until the mushrooms start to let out moisture and the herbs are fragrant (another 2-3 minutes)
Add the lentils, quinoa and barley (or whatever combo you're using) to the pan along with the vegetable stock, marmite and bay leaves.
Bring to a low boil and then simmer for 30-40 minutes, stirring regularly, until the grains have cooked. You may need to add more liquid throughout. It depends on your grains so just keep an eye on things.
Meanwhile, make the topping. Boil the potatoes and parsnips for 15-20 minutes until both are fully cooked. Drain and return to the pan with the goats cheese and butter. Using a hand mixer, whip the mixture until it's creamy and totally smooth.
Pre-heat the oven to 180C / 355F
Once the filling is fully cooked, add the frozen peas and cook for another few minutes until they are defrosted.
Assemble the pie. Pour the filling into a large oven dish (see notes below) and top with the mashed potato and parsnip mixture. Add the grated cheddar and the white parts of the spring onion to the top.
Cover and bake for 30-45 minutes.