Herbed Roast Pork

Name: Herbed Roast Pork
Ingredients: 16 Tbs. (2 sticks) (8 oz./225 g) unsalted European-style butter, at room temperature; 1 preserved lemon, homemade or a high-quality store-bought preserved lemon, rinsed and dried; Fleur de sel; 1 boneless pork loin roast, 3 to 3 1/2 lb. (1.4 to 1.5 kg); Fleur de sel and freshly ground pepper; 1 Tbs. extra-virgin olive oil, plus more for drizzling; Small handful of fresh sage leaves; 12 fresh rosemary sprigs, plus more for serving; 20 fresh thyme sprigs, plus more for serving; 3 fresh bay leaves; 2 garlic heads, halved across the equator; 1 medium yellow onion, quartered; 1 lemon, quartered