Preheat oven to 500F or 475F convection. Line two baking sheets with parchment. Divide broccoli and cauliflower between the baking sheets. Toss each with 1 tablespoon olive oil, making sure each floret is coated (use your hands to rub the oil into the florets). Bake for 10 minutes, or until nicely charred. If you want some extra char (highly recommend), hit it under the broiler for a minute or two. Remove from oven and sprinkle each sheet pan with ½ teaspoon salt and toss to coat.
Meanwhile, add all dressing ingredients to a high-speed blender and blend until totally smooth, about 1 minute. If too thick, add in water, 1 tablespoon at a time. You want it to be thick, but still “pour-able”. Taste and add more salt, if needed.
In a large bowl, add broccoli, cauliflower, beans, and celery. Pour 1 cup of dressing over top and toss to coat. Serve topped with a drizzle more of dressing (optional) and plenty of black pepper.