For the rub: Combine the cumin, paprika, coriander, dry mustard, onion powder, garlic powder, cayenne and some salt and pepper in a small bowl.
For the cauliflower: Preheat a grill pan or grill over medium heat.
Place the stem side of the cauliflower on the counter and slice vertically into four 1-inch-thick “planks” or steaks. The pieces from the center with the core in them work best for this as the core holds them together. Save any excess cauliflower or florets that fall off when cutting for another recipe.
Drizzle the cauliflower pieces on both sides with the olive oil, then sprinkle them generously with the spice rub, ensuring they are coated well.
Place the cauliflower steaks on the grill, cover with a sheet pan or similar cover to help retain heat and steam and cook until well-marked and just starting to lightly char in places, 7 to 8 minutes. Flip and cook until the cauliflower is crisp-tender, 7 to 8 more minutes.
Meanwhile, combine the Greek yogurt, oregano, mint, honey, lemon juice, garlic and salt and pepper to taste and mix well.
Drizzle the cauliflower steaks generously with the yogurt sauce. Garnish with mint leaves.