How to Make It
In a small saucepan, heat the vegetable oil. Add the garlic and cook over low heat until pale golden and crisp, stirring constantly, about 3 minutes. Drain the garlic chips on paper towels and reserve the oil; let both cool to room temperature.
Using a sharp paring knife, make 1-inch slits all over the tenderloin steaks. Stuff the slits with the garlic chips.
Fill a large, sturdy, resealable plastic freezer bag with the shallots, rosemary, thyme, lime leaves, orange zest, fish sauce, soy sauce, dried chiles and the reserved garlic oil. Add the steaks and seal, turning to coat the meat with the marinade. Let stand at room temperature for 2 hours.
Bring a large pot of water barely to a simmer; the water should register 135° on a candy or instant-read thermometer. Discard most of the marinade and reseal the plastic bag, pressing out any air. Add the bag to the water and cover with a rack or plate to keep it submerged. Cook the meat in the bag at 135° for 45 minutes, adjusting the heat as necessary to maintain the temperature. Remove the bag from the water. Transfer the steaks to a plate; scrape off the marinade.
In a skillet, melt the butter in the olive oil and heat until nearly smoking. Add the steaks and cook over high heat, turning once, until browned and an instant-read thermometer inserted in the thickest part of the steaks registers 130° for medium-rare, 6 minutes. Serve right away.