To prepare the mushrooms: slice them thinly, then toss with the olive oil, tamari, and lemon juice. Let them sit somewhere warm for 30 minutes, so they can marinate.
To make the soup base: blend all the ingredients until smooth. If you want it super smooth, strain through a cheesecloth or a nut milk bag.
Scoop the mushrooms evenly into 2 bowls, then pour the soup over them and garnish with parsley, dulse flakes, and sesame seeds.