Heat half of the butter in a frying pan. Saute the onion and garlic until aromatic, about a minute.
Add the mushrooms and the rest of the butter to the pan. Stir fry for half a minute. Pour in the liquid seasoning. Cook, stirring often, for about three minutes or until the mushrooms are done. DO NOT overcook mushrooms as they toughen and turn rubbery when exposed too long in the heat. If the mixture appears too dry before the mushrooms are done, add water, two tablespoonfuls at a time, to prevent scorching.
When the mushrooms are done, add the kalamansi juice. Stir fry for another 10 seconds.
Scoop out the mushrooms and transfer to a bowl.
Heat the remaining liquid in the pan. Add the shredded cabbage and stir fry with a generous pinch of salt just until very lightly softened.
Pile the cabbage on a plate. Arrange the mushrooms on top. Drizzle whatever remaining pan juices there are over the mushrooms. Sprinkle with scallions and sesame seeds before serving.