Place all ingredients for the harissa dressing in a blender and puree until smooth and creamy. Taste, adjusting for seasoning as needed. Set aside.
Preheat the oven to 400 degrees F. Toss the potatoes with 2 teaspoons olive oil and half the salt and pepper. Place in a single layer on a baking sheet or stone and cook until crispy, about 35-40 minutes.
Toss the broccoli and cauliflower florets with the remaining olive oil and salt/pepper and place in a single layer on a baking sheet or stone. If you don’t have enough trays, you may have to wait for the potatoes or tofu to finish cooking. Cook until just tender, about 15-20 minutes.
While the potatoes are cooking, press and drain the tofu. Slice into cubes then place on a lightly greased baking sheet. Place in the oven with the potatoes to brown and dry out, about 15 minutes.
Toss the cooked tofu with remaining harissa dressing.
Divide cauliflower, broccoli and potatoes among 4 bowls and drizzle with harissa tahini dressing.