Preheat oven to 425ºF. Brush 1 Tbsp. of the olive oil on a large rimmed baking sheet.Remove the outer green leaves from the cauliflower base. Use a sharp chef's knife to slice the stem off the cauliflower. Place the cauliflower stem side down on a cutting board and make one cut through the center to divide it in half. Cut each half into slices, about 3/4-inch thick. (You should get about 2 slices (or "steaks") per half, and the rest will break into florets.)Place slices and florets onto prepared baking sheet, and brush remaining 2 Tbsp. oil overtop. Season with 1/4 tsp. each of salt and black pepper.Bake for 40 minutes, carefully flipping once halfway through.
Meanwhile, combine tahini, whole-grain mustard, Dijon mustard, maple syrup, garlic powder, nutritional yeast (if using), and remaining 1/4 tsp. each of salt and pepper in a bowl; mix well. Add 2 Tbsp. of water and mix until smooth and creamy.Spread 1 Tbsp. of the tahini sauce on a plate or serving platter. Place cauliflower steaks and florets over the sauce, and drizzle remaining sauce overtop. Garnish with fresh chopped dill or parsley, and extra cracked black pepper.
Notes
Refrigerate leftovers up to 4 days. Rewarm in the microwave, or enjoy closer to room temperature.