Preheat oven to 425F/ 220C.
Trim leaves off cauliflower leaving the long stem in tact. Slice the very end off the stem. Slice the Cauliflower into 5-6 slices about ¾ inches thick each. Line a sheet pan with parchment paper. Brush the steaks on both sides with olive oil and place on the parchment.
Sprinkle with salt and pepper. Combine the cumin, coriander, paprika and oregano in a small bowl and sprinkle the cauliflower on both sides with this mixture. Roast in the middle of the oven until fork tender about 20-25 minutes.
While the cauliflower is roasting make the capers and green tahini sauce.
Heat the quarter cup of olive oil in a small frying pan over medium high heat and add the capers. Fry, stirring gently, for about 5 minutes, or until they puff, crisp and brown slightly. Remove and set aside.
Make the tahini sauce: Place all the ingredients except the tahini, water and lemon juice in the blender, and blend until combined to a paste. Add the tahini and, pulsing, add the lemon juice and water in alternating increments, until the tahini sauce is smooth and creamy. Adjust the flavour and texture with salt and additional lemon juice or water as needed.
Serve the cauliflower steaks drizzled with a little of the tahini sauce and garnished with a teaspoon of the crisped capers and olive oil from the pan. Serve warm.