Preheat a grill or a grill pan to medium-high. Drizzle the bread with EVOO and grill until toasted, 1 to 2 minutes on each side. Rub the garlic all over.
Place the tuna in a food processor and pulse until ground. Transfer the tuna to a bowl and add the rosemary, grill seasoning, orange zest, Worcestershire sauce and vinegar and a liberal drizzle of EVOO.
Brush the onion slices with EVOO and grill them until tender, 3 to 4 minutes on each side.
While the onions cook, ready the burgers to go on the grill by forming 4 large patties. Grill the patties with the lid down for 2 minutes on each side for rare and up to 5 minutes on each side for well-done. A minute before taking the burgers off the grill, top with an onion slice and 2 slices of cheese. Place the grilled bread on a platter, top with the orange slices, radicchio and burgers. Season with salt and pepper and enjoy!
