Position the racks in the upper and lower thirds of the oven; preheat to 450 degrees . For the cauliflower "steaks," trim off the stem, then cut about 1 inch from two opposite sides. Cutting from the top to the bottom, slice into 4 steaks, each 1 1/2 inches thick. (If serving as a side, halve each steak.) Arrange on nonstick or parchment paper-lined baking sheets. Spray or brush with olive oil; season both sides with salt and pepper. Roast on the lower rack 12 minutes. Turn each steak and top with a little cheese; roast until the cauliflower is tender and the top is golden-brown, 10 to 12 minutes more.
Scatter the nuts on a small baking sheet. Place in the oven on the upper rack and roast with the cauliflower until golden, 3 to 5 minutes; let cool.
In a small saucepan, bring the rum and raisins to a gentle simmer over medium heat to plump the raisins.
Meanwhile, in a food processor, finely chop the parsley, shallot, lemon zest and juice, capers, vinegar, garlic, anchovies and crushed red pepper. With the machine running, stream in the EVOO and process until smooth. Season the salsa verde.
Spoon the salsa over the steaks; top with the nuts and the drained raisins.