Heat the olive oil in a small saucepan, over a medium heat. Once hot, add the chopped portobello mushrooms and crushed garlic. Sauté for about 5 minutes until the mushrooms have soaked up the oil and are cooked through.
Add the cumin, coriander, and a few pinches of freshly ground pepper (to your taste). Stir through and allow to cook for a further 2 minutes until all the flavours come together.
Add ½ cup of the milk and mix through the flavours. Then add the rest of the milk, stir, and allow to come to boil.
In a small cup or measuring jug, add a few drops of cold water to the potato starch – or just enough so that the starch is fully dissolved in the water. Then add the starch to the saucepan and stir constantly as the milk simmers and thickens.
Lastly, add the salt! I like to add about 1 tsp of salt to balance out the sweetness of the milk, but you can add more or less to suit your personal taste. (Also, if you are using unsweetened milk, then you will need less salt.) Stir through well, then take off the heat and serve.
Notes
Cooking Tips: - Feel free to add a little bit more potato starch, if you'd like a thicker sauce.