Prepare a grill for high heat. Tear off two 18-inch-long pieces of aluminum foil.
Heat a frying pan over medium-high heat and toast the almonds, tossing occasionally, until lightly toasted.
Remove the bottom leaves of the cauliflower, and trim the stem down to the base. Turn the cauliflower on its side. Starting near the stem end, cut the cauliflower crosswise to make two 1-inch-thick rounds. (If you start near the top, the cauliflower will crumble.) Cut each round in half so you have 4 semicircles. Save any trimmings for another use.
Whisk together the harissa and 3 tablespoons of the olive oil in a small bowl. Place 2 cauliflower steaks on each piece of foil. Brush both sides of each steak with the harissa mixture, and sprinkle on a little salt. Wrap the steaks up to make foil packets as flat as possible, for maximum contact with the grill.
Place the 2 foil packets on the hot grill, sealed-side down, and grill for 12 minutes. Flip the packets, and open them up with tongs to allow the steam to escape. Cook the cauliflower in the opened foil until tender and nicely browned on the bottom, 6 to 8 minutes.
Meanwhile, heat 1 tablespoon of the oil in a medium saucepan over medium heat. Add the garlic and ginger, and stir for about 1 minute. Add the couscous, 2 1/4 cups water, the raisins and 1 teaspoon salt. Bring to a boil. Cover, reduce the heat to low and cook until all the water has been absorbed and the couscous is tender, about 12 minutes. Transfer the couscous to a large bowl, and stir with a fork to break up any clumps and release excess steam. Let sit for 5 minutes to cool slightly.
When ready to serve, add the remaining 3 tablespoons oil, the lemon juice and zest, feta, almonds, parsley, 1/2 teaspoon salt and some ground black pepper to the couscous. Divide the couscous and cauliflower steaks among 4 plates. Scatter 3 olives on each plate. Drizzle the cauliflower with oil, and add a squeeze of lemon juice.
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