Heat oven to 450°F.
Cut cabbage into ½-inch ribbons, then cut the ribbons into 1- to 2-inch segments.
Cut cauliflower into ½- to 1-inch chunks (i.e. quite small).
On a large baking sheet, toss with olive oil until lightly coated (about 2 to 3 tablespoons) and sprinkle with salt and pepper.
Roast for 10 minutes, then remove the pan from the oven just long enough to toss everything around so it colors evenly.
Return to the oven for 5 minutes, toss again, and 5 final minutes if needed. Don’t be afraid to get some serious char on the vegetables.
Meanwhile, make the dressing in a large bowl. Whisk garlic, lemon, tahini, and olive oil together, then whisk in water, 1 tablespoon at a time, until it reaches a thick dressing consistency.
Season well with salt and black pepper, or red pepper flakes. For more heat, you can add harissa directly to the dressing to taste.
As soon as the vegetables are crispy, charred, and ready, add them to the bowl with the dressing, toss to coat, and eat right away.
