How to Make It
In a large bowl, combine the balsamic vinegar, sherry vinegar and onion. Let stand at room temperature for 1 hour.
In another large bowl, cover the radicchio with ice water and let stand for 15 minutes. Drain and dry well.
Remove the onion from the vinegar; discard the onion. Whisk the honey and olive oil into the vinegar and add the radicchio and 1 cup of the Manchego. Season with salt and pepper and toss to coat evenly. Mound the radicchio on a serving platter, top with the remaining Manchego and serve.