Preheat the oven to 375 degrees F.
Steam the cauliflower over simmering water. Set aside.
Add the kale to the simmering water and cook until soft, about 10 minutes.Remove and drain well, squeezing most of the moisture out of the kale leaves.
Combine the yogurt, harissa, Worcestershire sauce, curry, sriracha, orange zest, orange juice and vinegar in a large bowl. Whisk to thoroughly combine. Add the shallots, tofu (if using), 1/2 cup cheddar cheese, and the cauliflower and kale. Toss together with a large metal spoon until everything is coated with the yogurt sauce.
Add the chopped tomatoes, and gently toss again.
Spoon the mixture into a medium casserole dish. Top with the sliced tomatoes and the breadcrumbs.
Bake for 30 minutes, or until starting to bubble. Remove the gratin from the oven and sprinkle the rest of the cheddar cheese over the top. Return the gratin to the oven and bake for another 10 minutes.