Make the green garlic mojo Wash the garlic well and split in half, lengthwise. Cut crosswise into thin slices (you’ll need about 1 1/4 cups) and scoop into a medium (3-quart) saucepan. Set over medium-low heat and measure in the oil and 3/4 teaspoon salt. Stir occasionally as the mixture comes barely to a simmer (there should be just a hint of movement on the surface of the oil). Adjust the heat to the very lowest possible setting to keep the mixture at that very gentle simmer (bubbles will rise in the pot like mineral water) and cook, stirring occasionally, until the garlic is soft, about 15 to 20 minutes (the slower the cooking, the sweeter the garlic). Season with lime juice, epazote and lemon thyme. Continue simmering for 10 minutes to allow all the flavors to come together. Then taste the mojo de ajo and add a little more salt if you think it needs it.
Grilling the lamb Light a charcoal fire and let the coals burn until covered with gray ash but still hot; bank the coals to one side and set the grill grate in place. Or, heat one side of a gas grill to medium-high.
Rub both sides of the lamb with the green garlic mojo and sprinkle with salt and black pepper. Place the lamb on the grill, fat cap side up, and cover. After 15 minutes, flip the lamb and cook for 15 minutes more, depending on the heat of your fire. When the lamb reaches an internal temperature of 130 degrees, remove from the heat and let rest in a warm place. The meat will come to its serving temperature (I think 135 degrees is ideal) after about 10 minutes of resting.
To serve, cut the lamb into slices about 1-inch thick and top with more of the warmed mojo.