1. Finely chop the garlic and the ginger, cut the tomatoes into small pieces and dice the mushrooms into big chunks.
2. Peel and dice the potatoes, into roughly one centimetre (half an inch) cubes.
3. Take a pot, tip in the water and turn on the heat on medium.
4. Tip in the ginger, the garlic, and the mustard seeds, and stir well to combine.
5. Bring the liquid to a boil and cook the ingredients until they are soft and translucent.
6. Add in the tomatoes and cook them until they soften a bit, roughly 4-5 minutes.
7. Add in the mushrooms, the paprika, some black pepper and stir to combine the ingredients. Cook for another 5 minutes, making sure to keep an eye on the pot and to stir frequently.
8. Once the time has elapsed, add in the potatoes and stir.
9. Add the tomato sauce, the water (or stock) and stir again to properly combine all the ingredients together. Bring the liquid to a boil, cover the pot with a lid, lower the heat to the minimum and cook for the ingredients for 15 minutes.
10. Once the time has elapsed, throw in the peas, and increase the heat to medium. Cook the stew for another 10 minutes so that the potatoes have the time to properly soften, and the liquid to reduce hence thickening the stew.
11. For a thicker stew, cook it for another 5 minutes to reduce it even more, or crush a few of the potatoes with a fork, releasing some of their starch and naturally thickening the sauce. Remember to taste the stew and adjust salt if necessary.

http://www.thevegancorner.com/mushroom-and-potato-stew/