Heat oven to 450°F. 

Cut the cabbage into 1/2-inch ribbons, then cut the ribbons into 1- to 2-inch segments.  Cut cauliflower into 1/2- to 1-inch chunks (i.e. quite small).

On a large baking sheet, toss the cauliflower and cabbage with olive oil until lightly coated (about 2 to 3 tablespoons), and sprinkle with the salt and pepper. I used two baking sheets, to make sure everything had room to breathe and not get soggy.

Roast for 15 minutes, then toss everything around so it colors evenly.  Return to the oven for 5 minutes, toss again, and 5 final minutes if needed.  I'll be honest though, sometimes I need even another 5 or 10 minutes after that.  Don’t be afraid to get some serious char on the vegetables. They will taste good, not burnt.

While the veggies are roasting, make the dressing in a medium bowl. Whisk the garlic, lemon juice, tahini, and olive oil together, then whisk in some water, 1 tablespoon at a time, until it reaches a thick dressing consistency (usually 2-3 tablespoons works well). 

Season the dressing well with a strong spinkle of harissa (if you have it; mine is a spice mix, not a paste), salt, black pepper, and red pepper flakes to taste.  Taste and add any additional salt as necessary as well.

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.  This makes pretty great leftovers as well.