Step 1Place the mushrooms in a bowl with hot water to cover, then set a plate over the bowl to prevent the steam from escaping. Let stand until the mushrooms rehydrate and soften, about 30 minutes (or longer depending on the size of the mushrooms). Squeeze the mushrooms between the palms of your hands to get rid of the excess water. Using a paring knife, remove any hard stems from the mushrooms. Keep the caps whole or halve or julienne them.
Step 2Trim and remove the ends of the greens. Cut the remaining stems and leaves into 1 1/2 -inch long pieces. Trim the stem end of the bok choy and separate the leaves. If using baby bok choy, keep the leaves whole; if bigger, halve them lengthwise.
Step 3Set a bamboo steamer with a lid over a wok filled halfway with water and bring the water to a boil over high heat. When the steamer is filled with hot steam, place a plate in the bottom of the basket and add the leafy greens on top of the plate. Secure the steamer lid on top and steam the greens until cooked through, about 10 minutes.
Step 4Meanwhile, heat the oil in a second wok over high heat. Stir-fry the garlic until fragrant and golden, about 1 minute. Stir in the stock, wine, oyster sauce, sesame oil and sugar. Allow the sauce to come to a boil, then reduce the heat to medium-low, add the mushrooms and cook, covered, until tender, 5 to 7 minutes. Adjust seasoning to taste with salt and pepper. Dissolve the starch in a tablespoon of water and stir it into the sauce. Boil until the sauce has thickened, about 1 to 3 minutes.
Step 5Remove the leafy greens from the steamer and place in a deep serving platter. Pour the sauce with mushrooms over the greens and serve garnished with cilantro.