Preheat a grill to medium high. Combine the hoisin sauce, ketchup, vinegar, ginger, honey and sambal oelek in a small bowl. Season the steak with salt and pepper. Brush about half of the hoisin sauce mixture all over the steak.
Put the cauliflower and 1 tablespoon water in a large microwave-safe bowl, cover and microwave until the cauliflower is crisp-tender, 5 minutes. Blot dry. Toss the cauliflower and mushrooms with the vegetable oil and 3/4 teaspoon salt; spread out in a grill basket.
Oil the grill grates. Grill the steaks until well marked, 3 to 4 minutes per side for medium rare, brushing with the remaining hoisin sauce mixture during the last minute of cooking. Transfer to a cutting board to rest. Meanwhile, grill the vegetables, tossing occasionally, until browned around the edges and crisp-tender, about 8 minutes, adding the larger scallion pieces to the grill basket during the last 2 minutes of cooking.
Divide the vegetables among plates. Slice the steaks, add to the plates and sprinkle with the thinly sliced scallions.