Preheat the oven to 450°F (230°C). Adjust a rack to the lower third of the oven. Add 2 quarts of water to a medium saucepan with a lid. Bring to a boil, uncovered, over high heat.
Meanwhile, on a cutting board, halve the cauliflower through its stalk. Cutting the same direction, cut a (1-inch-thick) “steak” from the cut side of each cauliflower half so you have 2 cauliflower steaks plus remaining stemless florets from the rounded side of each half. If any of the steak florets separate, don’t worry, just keep them together as much as possible.
Place the 2 steaks on a rimmed baking sheet. Coarsely chop the remaining florets and set aside.
Carefully baste the steaks on both sides with one quarter of the olive oil, generously salting both sides along the way. Roast until the underside is deep golden brown, 15 to 18 minutes, flip the steaks and roast 5 minutes more.
Season the boiling water generously with the salt, add the chopped cauliflower, and boil until very tender, 8 to 10 minutes. Drain the cauliflower and place it in a food processor fitted with a steel blade. Add the almond milk and one quarter of the olive oil. Puree the cauliflower until smooth. Season to taste with salt and pepper.
In a small bowl, combine the remaining olive oil and the capers, almonds, and vinegar. Stir to combine and season with salt and pepper to taste.
Divide the cauliflower purée between dinner plates. Top each with a cauliflower steak and spoonfuls of the caper sauce. Sprinkle with parsley leaves and serve.