To make the shiitake “bacon” crisps, preheat the oven to 375ºF. Place the sliced mushrooms on a baking tray lined with parchment paper. Drizzle about 2 tablespoons of oil on top and sprinkle liberally with salt. Toss with your fingers to combine. Spread the mushrooms evenly in a single layer. Place in the oven and bake for 20 to 25 minutes until they look almost burnt. Remove from the oven and set aside to cool and crisp up.
To blanch the broccoli and cauliflower, set a large pot of salted water on the stove to boil. Prepare an ice bath by filling a large bowl with about 75% ice, 25% water. Place the cauliflower in the boiling water for 1 minute. Use a slotted spoon to transfer to the ice bath. Repeat with the broccoli then add to the ice bath with the cauliflower. Drain the vegetables well and give them a good shake in the strainer to remove as much excess water as possible.
To make the dressing, mix the tahini, yogurt, ginger, garlic, lime juice, vinegar, syrup, salt, and red pepper flakes in a large bowl. Transfer the cauliflower and broccoli to the bowl with the dressing and toss to combine. Stir in walnuts and scallions. Add ground pepper. Chill until ready to serve. Just before serving, sprinkle shiitake “bacon” on top of each serving.