Creamy Shrimp Risotto with Mascarpone

Name: Creamy Shrimp Risotto with Mascarpone
Ingredients: 4 Tbsp. extra-virgin olive oil, divided; 1 lb. head-on shrimp, shells and heads removed and reserved, shrimp cut into 1 pieces; 2 Tbsp. tomato paste; ⅔ cup dry white wine, divided; 1 medium carrot, scrubbed, halved crosswise; 1 celery stalk, halved crosswise; 6 large sprigs thyme; 1 tsp. whole black peppercorns; 2 large onions, peeled, halved through root end, divided; 1 head of garlic, cloves separated, peeled, divided; Kosher salt; 1½ cups arborio rice; Freshly ground black pepper; 1 lemon; 2 Tbsp. unsalted butter; 6 oz. mascarpone, room temperature, divided; 2 Tbsp. thinly sliced chives