Place the first cauliflower upright (stem-side down) on a flat surface. Using a large knife trim a little off each end (sides) so that when you slit the cauliflower in half, each half will lie flat. Cut the Cauliflower in half. You will yield 2 steaks about 14 ounces each. Repeat with the other cauliflower. If you want thinner steaks, cut each half into halves.
Bring about 6 quarts of water to a boil in a pot large enough to hole the cauliflower steaks. Add salt to the water and check for saltiness. You want it to taste like seawater. Layer a sheet pan with paper towel. Add the cauliflower steaks to the boiling water and cook until they are slightly tender when pierced with the tip of a knife, 6 to 8 minutes. Using a slotted spoon or spatula carefully transfer the steaks to the sheet pan to drain.
In a small bowl whisk together the extra virgin olive oil, the lemon zest, the lemon juice, and the za'atar. Remove the paper towel from the sheet pan insuring the cauliflower steaks are dry. then, pour the mixture over each steak turning to make sure each steak is generously covered. Cover and let marinate at least 30 minutes.
Heat a grill or your inside grill pan to high. Place the steaks on the grill and cook until each side is a little charred, being careful to not break up the cauliflower.
Place on a serving platter and top with the Tomato and olive relish. Drizzle with a little more olive oil if you like. Garnish with a little fresh parsley.