Preheat the oven to 425°F. Trim the base of each cauliflower and remove the leaves while leaving the stalk intact. Cut each cauliflower in half. Cut a 1-inch thick steak from each half to make 4 steaks total. Pull the remaining cauliflower apart. If there are any other large florets, they can be trimmed into small 1-inch thick steaks. Reserve the remaining cauliflower for the purée – there should be about 4 cups of florets.
In a small bowl, combine the Dijon mustard, lemon juice, kosher salt, granulated garlic, granulated onion, dried dill, and pepper. Whisk together. Add the olive oil and whisk until smooth. Spray a baking sheet with non-stick spray. Arrange the cauliflower steaks on the baking sheet. Use a pastry brush to coat both sides of the cauliflower with the marinade.
Cover the baking sheet with foil and roast for 10 minutes. Remove the foil and continue roasting for another 10 minutes. Carefully flip the cauliflower and roast for another 10 minutes. Flip the cauliflower again and roast for another 5 to 10 minutes until the steaks are tender and golden brown.
Cauliflower Purée
Break down the remaining cauliflower florets into uniformly sized pieces. Add the cauliflower to a steamer and cook for 10 to 15 minutes until tender. Transfer the cauliflower to a food processor and add the cream, butter, kosher salt, and pepper. Blend the cauliflower into a smooth purée. Add the shredded cheese and blend until smooth. Keep warm until ready to serve.
Horseradish Crumbs
In a non-stick skillet, melt the butter over medium heat. Add the minced garlic and prepared horseradish. Sauté for about 30 seconds until fragrant. Add the panko crumbs, stirring to fully the coat the crumbs with the butter. Season with the kosher salt and pepper. Cook the crumbs, stirring regularly, until golden brown. Once browned, remove from the heat and stir in the fresh dill and parsley. Set aside until ready to serve.
To serve, spoon some of the purée onto each plate. Arrange the roasted cauliflower steaks on top of the purée and top with the horseradish crumbs. Garnish with a bit more chopped fresh dill and parsley. Enjoy!