Melt the first portion of butter in a 12-inch skillet over medium heat. Arrange the fennel slabs in the pan—it will look crowded, but they will shrink as they cook—then add the lemon zest, salt, pepper, and wine. Cover the pan and reduce the heat to medium-low, simmering for 15 minutes.

Flip the fennel pieces and replace the cover. Cook until the fennel is tender and deeply browned, about 7 minutes. Flip the pieces once more and continue to cook, covered, until well browned on the second side, about 7 minutes. Transfer the fennel to a platter and tent loosely with foil to keep it warm.

Add the radicchio, water, and honey to the liquid remaining in the skillet. Cook over low heat, scraping the bottom of the pan to release the browned bits, until the leaves are wilted, 3 to 5 minutes.

Whisk in the lemon juice and the remaining butter and cook until the sauce thickens slightly, about 1 minute. Season with salt and pepper to taste.

Pour the radicchio and sauce over the fennel on the platter. Finish by sprinkling with pine nuts, reserved fennel fronds, and shaved Parmesan, and serve while warm.