Grilled Turkey Breast with Caramelized Onion, Cracked Black Pepper, and Vinegar

Ingredients

• 8 tbsp. unsalted butter, cubed and chilled
• 2 medium red onions, finely chopped
• Kosher salt, to taste
• 1 cup white vinegar
• 2 cups turkey or chicken stock
• 1 tbsp. cracked black peppercorns, plus freshly ground black pepper, to taste
• 2 ¹⁄₄ turkey breast, cut into 4" pieces and pounded into 1/6–thick medallions
• 2 tbsp. olive oil