Heat oven to 200˚C.
Line a large baking tray with baking paper.
Bring stock to the boil in a large saucepan.
Slowly sprinkle in the polenta, whisking while you add it.
Bring to the boil and then simmer for 8-10 minutes, stirring frequently.
Remove from the heat and taste to adjust the seasoning.
If your stock is not very salty, add more with a good amount of freshly ground black pepper.
Spread the polenta onto the lined baking tray.
You want the tart base about 2cm thick.
Bake for 30 minutes.
Meanwhile, cook the mushrooms.
Heat a drizzle of olive oil in a large frying pan.
Add the mushrooms, thyme and rosemary.
Cook until golden brown.
Season with salt and pepper.
Remove the polenta from the oven.
Top with some dollops of vegan cheese followed by the mushrooms.
Bake again for 10 minutes.
Remove from the oven and plate on a board.
Toss the rocket with the balsamic vinegar.
Scatter over the tart and serve!