Place the loin pieces in a 9x13x2-inch baking dish and rub all over with the mustard mix. Heat the butter in a large skillet over high heat. Once melted, sear the loin pieces in batches, until golden, about 1–2 minutes per side. Transfer back to the baking dish. Arrange the potatoes, garlic halves and thyme stems around the seared loin pieces. Drizzle the potatoes and garlic with olive oil and place the dish on the middle rack of the oven. Roast until the loin pieces read 145 °F (62.8 °C) with a thermometer inserted into the center of the thickest end, about 30 minutes. Transfer the loin roasts to a cutting board and tent with foil until ready to serve.