Pour about 1 inch of water into a large saucepan or pot. Fit a steamer basket in the pan. Cut the cauliflower into quarters. Remove and discard the core. Place the cauliflower in the steamer, cover, and place over medium-high heat. Steam until almost but not quite tender, 10 to 12 minutes. Remove from heat and let cool. Chop the cauliflower into 1/2-inch chunks and transfer to a large bowl. Add the oil, raisins, capers, salt, pepper, and (if using) vinegar and caraway seeds. Let stand for 15 minutes. (You can cover the relish and refrigerate it for up to 5 days.)
Place the bread slices on a work surface. Spread half the slices with the mustard, then top with the salami and the cauliflower relish. Sandwich with the remaining bread.