Preheat the oven to 400°F. Trim leaves from cauliflower, leaving the stem intact. Cut cauliflower vertically from top through stem. Cut each half into two 3/4-inch slices, saving any florets or large pieces that remain.
Place cauliflower on a sheet pan and drizzle both sides with olive oil.
Combine Rub ingredients; sprinkle 1/2 teaspoon over both sides of cauliflower slices. Bake 40 minutes or until cauliflower is tender.
After 15 minutes, heat a medium ovenproof skillet over medium-high heat until hot. Coat steaks on both sides with remaining 2-1/2 teaspoons rub. Brown steaks on both sides in skillet. Transfer skillet to oven; cook about 15 minutes for medium rare 145°F doneness.
Carve steaks into thin slices. Serve steak slices on cauliflower steaks. Drizzle with prepared horseradish sauce.
Nutrition information per serving: 298 Calories; 144 kcal Calories from fat; 16 g Total Fat; 4 g Saturated Fat; 8 g Monounsaturated Fat; 87 mg Cholesterol; 729 mg Sodium; 10 g Total Carbohydrate; 3.2 g Dietary Fiber; 29 g Protein; 3.7 mg Iron; 5.3 mg Niacin; 0.7 mg Vitamin B6; 3.9 mcg Vitamin B12; 4 mg Zinc; 23.9 mcg Selenium; 70.2 mg Choline;