In a medium bowl, whisk all of your marinade ingredients together until combined; transfer to a resealable plastic bag, along with the pork tenderloin. Toss to coat. Marinate in the refrigerator overnight, then discard marinade.
Prepare a grill to medium-high heat.
Oil grill grates and grill tenderloin, turning every few minutes on all sides, until a thermometer inserted into meat registers 145°, about 15 minutes. Transfer to a cutting board and tent with foil; let rest 10 minutes.
While meat is resting, grill the onions. Drizzle each slice with olive oil and season both sides with salt and pepper, to taste. Grill for 3 to 5 minutes on each side until onion is tender and has begun to caramelize.
Cut tenderloin into thin slices and serve with the grilled onion and a dab of herb butter.