Pre-heat oven to 180°C.
Peel and finely dice the red onions and garlic.
Finely dice the carrot and celery.
Chop sun-dried tomatoes, rosemary and thyme.
Blitz mushrooms to a fine mince in food processor.
Heat skillet over medium heat and add sun-dried tomatoes and oil, a pinch of salt and onion. Fry until onions become translucent, around 5 minutes.
Add the garlic, sun-dried tomatoes, rosemary and thyme. Cook for 2 minutes. Then add the carrot and celery and cook for a further 5 minutes, while stirring.
Add mushrooms and cook for 5 - 7 minutes, stirring occasionally.
Stir in tomato paste, vegemite and balsamic vinegar until combined. Add in red wine, stock and lentils and simmer until most of the liquid has evaporated, around 10 minutes.
Season to taste and remove from heat.
Mash the potatoes with the dairy-free butter, milk and mustard. Opt for a smooth mash or chunky, rustic mash.
Add the filling to the baking dish in an even layer. Top with potato mash. If using a smooth mash, run a fork over the top of the mash to create an uneven texture. The potato peaks will brown nicely in the oven.
Put Shepherd's Pie in oven for around 20 - 30 minutes, until the tops of the potato are nicely browned.