Heat a large pot of water until it registers 134° on a digital probe or candy thermometer. Season the roast generously with salt and pepper. Rub all over with the thyme and garlic and transfer to a large, BPA-free resealable freezer bag. Add the butter and seal all but 1 corner. Gradually lower into a large bowl of water until nearly all of the air is pressed out, then seal. Add the bag with the roast to the pot and cook at 134° until an instant-read thermometer inserted in the thickest part registers 130°, about 1 hour and 45 minutes; reseal the bag using the same procedure as above, if necessary. Adjust the heat as needed to maintain the water temperature. Transfer the roast to a cutting board and let rest for 5 minutes. Scrape off and discard the thyme and garlic. Light a grill or preheat a skillet. Pat the roast dry with paper towels, then brush with oil and season lightly with salt and pepper. Cook the roast over very high heat, turning once, until nicely browned all over, 5 to 7 minutes. Return the roast to the cutting board and let rest for another 10 minutes. Thinly slice the meat against the grain and serve.