Preheat the oven to 425 degrees. Generously spray a baking sheet with cooking spray.
Cut cauliflower into 1 1/2-inch slices. Place on baking sheet. In a small bowl, combine cumin, coriander, turmeric, salt and pepper.
Drizzle cauliflower with oil. Rub with spice mixture. Roast for 15 minutes then flip slices and continue cooking for another 15 minutes until golden brown and slightly charred.
While roasting the cauliflower, add potatoes to another baking sheet and bake for 50 to 60 minutes, until easily pierced with a fork.
In a skillet over medium heat, add butter. When melted add the shallot and garlic and saute for 3 minutes. Add the capers, salt, pepper and stock. Bring to simmer and allow liquid to reduce slightly. Turn off heat. Stir in lemon juice, zest, and parsley.
Cut slits on top of hot baked potatoes. Add butter to each potato. Pour sauce over steaks and potatoes. Serve immediately.