First, remove the stems and leaves from the cauliflower. Slice each of the heads down the middle, then slide your knife about 1 ½ - 2 inches to the left and cut again. Do the same to the right. This leaves you with 2 steaks from each head. (The leftover cauliflower can be used in other recipes.)
Place the cauliflower steaks on a baking sheet and drizzle olive oil over them, Then sprinkle the ground flax seeds, Za'atar, salt, and onion powder on top. Tap the seasonings into place so they stick to the cauliflower.
Bake them at 375 degrees Fahrenheit for 15 minutes. Then flip them carefully over and bake for another 5-10 minutes. Broil them for 2 minutes to get a golden brown crust on top, and serve with creamy tahini sauce.
Creamy Tahini Sauce
½ cup tahini
¼ cup water, or more for a thinner consistency
¼ cup lemon juice, from about 2 freshly squeezed lemons
2 garlic cloves, minced
½ tsp sea salt
¼ tsp cumin
Mix together all of the ingredients above until smooth.