Coat a large saucepan with olive oil. Add the onions and bring to a medium heat. Add a generous pinch of salt and a small pinch of crushed red pepper. Cook for 5 to 7 minutes or until the onions look wilted and cooked but do not have any color. Add the garlic and cook for another 2 to 3 minutes. Stir in the tomatoes and 3/4 of a can of water, and season with salt. Bring the mixture to a boil, reduce to a simmer and cook for 20 to 30 minutes. Taste, it should taste good.
Bring a pot of water to a boil over medium heat and season it generously with salt. It should taste like the ocean. Add the cauliflower, let the water come to a rolling boil and cook the cauliflower for an additional 5 to 7 minutes. When done it should be really soft and almost falling apart. Strain the cauliflower and add it to the tomato mixture. Cook the cauliflower in the tomato sauce until the cauliflower has completely broken up and the sauce clings to the cauliflower, about 20 to 30 minutes. Taste to see if the seasoning needs to be adjusted. Stir in the lemon zest, olives and caperberries. Reserve. (This can totally be done ahead, it actually gets better when it IS done ahead. But it is still really good when made to order.)
For the fish:
Preheat the grill to medium and brush with oil if needed to remove any food particles. Pat the fish skin dry to make sure it is really dry. If you have time, let it hang out to dry for a few minutes before you grill it. Drizzle with olive oil and season with salt on both sides. Put the fish, skin side down, on the preheated grill. Cook the fish for 2 to 3 minutes and then rotate the fish 90 degrees to create crisscross grill marks. Cook for another 2 to 3 minutes and then turn the fish over and cook for another 2 minutes.
Reheat the cauliflower if it was done ahead. Serve the fish on serving plates, leaning on a mound of the cauliflower. In a small bowl toss the parsley with the lemon juice, salt and big fat drizzle of finishing oil. Garnish the fish with the parsley salad and more finishing oil, if desired.