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 seasoning, pancetta, and cheese into ground beef. Form 1/3 lb. patties with meat mixture.
Grill onions over medium-high heat until softened and lightly browned on both sides.
Grill burgers over medium heat until seared and registering 160°F, about 4 minutes each side. 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.