Add the frozen vegetables, diced potato, and vegetable stock to a large pan, and bring to a simmer. Cook until the potato is very soft - around 10-15 minutes.
Use a hand blender to carefully blitz the mixture until it's completely smooth (or as smooth as you want it!). Add the remaining ingredients (or whatever combination of herbs and spices you prefer), and season to taste. Simmer until the gravy reaches your desired consistency (add a dash more water if it ends up too thick). Serve warm.
Feel free to change up the herbs and spices depending on what you have in the cupboard - I'll probably never make this vegetable gravy the same way twice!