Toss together sweet potatoes, cauliflower, 3 tablespoons olive oil, salt and pepper on a rimmed baking sheet. Roast at 425 degrees F, tossing once, until golden, 25 to 30 minutes; cool. Whisk together remaining olive oil, sherry vinegar, salt and pepper in a bowl. Add lettuces, pomegranate seeds, and roasted vegetables; toss to coat. Serve immediately.