Lemony Sheet Pan Chicken with Potatoes and Cauliflower

• 2 lbbone-in, skin-on chicken breasts — split, 4
• ⅜ cupextra virgin olive oil — divided
• ¼ cupfresh lemon juice
• 1 tspDjion mustard
• 3large cloves garlic — minced
• 2 tbspdried oregano
• 2 tspcoarse kosher salt — divided
• 4 cupslarge head cauliflower — chopped into florets, ½ head
• 1½ lbYukon gold potatoes — diced
• ½lemon — thinly sliced
• ¼ cupfresh parsley — chopped
• —Greek Yogurt Tahini Sauce

Quick pickled onions

• ½red onion
• 2 tbspred wine vinegar
• ¾ tspkosher salt
• 1½ tspsumac — optional