Chop or slice your mushrooms, mushrooms of choice for me is Shimeji but you can use any mushrooms you like. I chopped about ⅓ of a 150 gram package.
On 4 April 2019, I used 100 grams of fresh Shiitake mushrooms, chopped, perfect! (I like a lot of mushrooms in the soup.)
Put everything except mushrooms in a microwave safe bowl, Pyrex or ceramic would be perfect, not a plastic bowl, and whisk together, then mix in the mushrooms.
Microwave on HIGH for 3 minutes, whisking well after 2 minutes and again at the end of cooking time. If not thick enough, add 30 seconds and whisk again. If needed, continue for 30 seconds at a time and whisking after each 30 seconds.
This makes just a bit more than a standard can of condensed soup.