In a large bowl, combine the onion, breadcrumbs, milk, Worcestershire sauce, salt, pepper, and sage. Let sit until the liquid is fully absorbed, about 5 minutes. Gently mix the pork and bacon into the breadcrumb mixture.
Divide the mixture into 6 balls (about 6 ounces each), being careful not to overwork the meat. Press into 1-inch thick patties (about 4 ½-inch rounds).
Preheat the grill to medium-high heat, 400°F to 425°F.
Oil the grill grates with the vegetable oil. Grill the patties over direct heat, covered, until the internal temperature reads 165°F, 5 to 6 minutes per side. If you like cheese on your burgers, add it during the last minute of cooking, then cover to melt.
Alternatively, cook on the stovetop: Heat the vegetable oil in a large cast-iron skillet or grill pan over medium-high heat. Working in batches as necessary, cook the patties until the bottoms are golden and crusty, 5 to 6 minutes. Flip, and cook until the internal temperature of the patties reads 165°F, about 3 minutes more. If you like cheese on your burgers, add it during the last minute of cooking, then cover to melt.
Serve the burgers on the toasted buns with desired toppings.