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 cool.
Chop the cauliflower into ½-inch chunks; transfer to a large bowl. Add the oil, raisins, capers, 1 teaspoon salt, ¼ teaspoon pepper, and (if desired) vinegar and caraway seeds. Let stand for 5 minutes.
Place the bread slices on a work surface. Spread half the slices with the mustard; top with the salami and cauliflower relish. Sandwich with the remaining bread.