Peel potatoes; cut off tops and bottoms to make flat ends. Cut potatoes in half crosswise to make 2 (2-inch tall) pieces from each potato. Sprinkle 1 end of potatoes with salt. Heat oil and butter in a large, 12-inch cast-iron skillet over medium-high. Add potatoes, standing them on their salted end. Cook, basting with butter mixture periodically, until well browned, 8 to 9 minutes. Flip potatoes and cook other end, basting periodically, until browned, 8 to 9 minutes. Transfer potatoes to a plate. Preheat oven to 375°F. Add onion, garlic, and thyme to skillet over medium. Cook, stirring occasionally, until onion is tender, about 3 minutes. Add flour; cook, stirring constantly for 30 seconds. Stir in cream, and bring to a boil for 2 minutes. Stir in cheese, about 1/2 cup at a time, stirring to melt before adding more. Return potatoes to skillet, standing them on their end; cover with aluminum foil. Bake in preheated oven until potatoes are tender, about 45 minutes. Sprinkle with chives and pepper, serve with sauce.