The panino con radicchio di Treviso is built on a single bitter leaf and the heat used to tame it. Radicchio di Treviso IGP is the elongated kind: long tapered heads with a white spine and deep wine-red blades, firmer and more pointedly bitter than the round red chicories. The defining move is that it is usually grilled rather than left raw. Heat collapses the leaf, caramelises its edges, and turns a sharp vegetable faintly sweet while keeping the bitter spine as a backbone. The sandwich is decided at the grill: a raw Treviso head would dominate the bread, so it is cooked first and the bitterness is brought down to something a panino can carry.
The craft is the char and what is set against it. The heads are halved or quartered lengthwise, brushed with oil, and laid on a hot grill until the cut faces blister and the leaves wilt without going to ash, the bitter edge softening as the sugars catch. That managed bitterness wants fat and salt opposite it, so the classic counter is a soft cheese, stracchino or a young taleggio, or a few slivers of speck whose smoke and salt answer the char. The bread is plain and sturdy, toasted lightly so it firms against the oil the radicchio sheds; the grilled leaf is laid in while it still holds warmth so the cheese just begins to slacken. The point is restraint: one bitter vegetable, cooked correctly, and one fatty thing to balance it.
The variations follow the leaf and its partners, each its own preparation rather than a footnote here: grilled Treviso with speck; the cheese build with stracchino or taleggio; and the late-bitter version dressed only with oil and salt for the cooked leaf to stand alone. Each is the same grilled-Treviso logic given a loaf, and each deserves its own article rather than being crowded in here.