Slice onion to create 1/4" discs, drizzle with olive oil and season with salt and pepper; set aside. Lightly butter the buns and set aside.
Gently mix the seasoning, pancetta, and cheese into ground beef. Form 1/3 pound patties with the meat mixture and press a slight dimple depression in the center of each burger. Allow to rest for 40 minutes while lighting the grill and cooking the onions.
Grill onions over medium-high heat until softened and browned on both sides.
Grill the burgers over med-high heat (dimple side up) for about 4 minutes. You'll see the color of the burger changing on the outside edge and juices begin to collect in the dimple. Turn and cook on the other side another 4-5 minutes, or until the internal temperature is between 155-160°F. While the burgers finish, grill buttered buns (butter side down) until toasted.
Serve the burger on the toasted bun with the grilled onions piled on top.