Pork Tenderloin with Apples and Onions
1 cup chicken stock 1 tablespoon butter 1 tablespoon Dijon mustard 1 tablespoon fresh thyme leaves, divided 1 teaspoon sea salt 2 (1 1/2 pound) pork tenderloins 2 medium sweet onions, sliced vertically 2 tablespoons vegetable oil, divided, or more… 2 teaspoons vegetable oil, divided 3 Granny Smith apples - peeled, cored, and sl… ¼ teaspoon ground black pepper, or to taste