Remove tough stems and ribs from kale; coarsely chop the greens. Cook the kale in 1/4 cup water in a large skillet over medium-high heat, covered, until barely tender, 2 to 3 minutes. Transfer to a colander. Cook broccoli/cauliflower the same way with the remaining 1/4 cup water. Transfer the kale to a large bowl; drain the broccoli/cauliflower in the colander. Wipe the pan dry.

Heat 1 tablespoon oil in the pan over medium-high heat. Add the kale and cook, stirring often, until tender and browned in spots, 4 to 6 minutes. Transfer to the bowl. Heat the remaining 1 tablespoon oil in the pan. Add the broccoli and cook, stirring often, until tender and browned in spots, 4 to 6 minutes. Transfer to the bowl.

Heat butter, garlic and crushed red pepper in the pan over medium heat until the butter is melted. Cook, stirring constantly, until the garlic is light brown, 1 to 2 minutes. Drizzle the butter over the vegetables and sprinkle with 1/4 teaspoon salt; gently toss to combine. Serve topped with a sprinkling of Maldon sea salt and crushed red pepper, if desired.