Prepare the maitake by removing the stem, trimming, and cutting into ¼ inch thick slices. If using shiitake, quarter if large and halve if medium. Prepare the cauliflower: separate into florets, square the stems and slice.
Fill a large bowl with water and ice cubes. Bring abundant salted water to a boil in a wok. Add the cauliflower and blanch for 30 seconds, drain, and transfer it to the ice water. When the cauliflower is cold, drain it, transfer it to a plate, and set aside.
Dry the wok, add the butter, and heat it over high heat. When the butter has melted, add the panko and stir-fry gently until the panko is golden brown, about 1 minute. Watch carefully to avoid burning. Transfer the panko to a medium bowl.
Wipe out the wok, and heat over high heat. Add the oil and swirl to coat the pan. When the oil is hot, add the garlic, ginger and maitake and stir-fry until soften, about 2 minutes. Add the cauliflower, oyster sauce and lemon zest and juice. Season with salt and pepper and stir to heat through, 2 to 3 minutes.
Make a bed of the rice on a platter, or transfer to four individual serving bowls, and top with the stir-fry. Sprinkle with the panko, garnish with the chives and serve.